Check to make sure nothing is stuck in the chamber or in the barrel. Make sure your cleaning rod isn't getting stuck on the side of the chamber or in the throat of the barrel. Make sure you have the right size brush attached. Sounds to me like it is probably very simple that is causing this, but without actually seeing what is going on, it is hard to pinpoint the exact cause.
Here are a couple links that might help you with the process:
Cleaning and Maintenance of the AR-15
Safety Tips for an AR-15 Rifle: Cleaning & Disassembling an AR-15 Rifle | eHow.com
Field stripping the AR15 (massive pics) - AR15.COM
:: Maintenance Guides
Those should get you started to a well-maintained AR.
As for the extractor upgrades, check out Bravo Company's kit:
BCM SOPMOD Bolt Upgrade Rebuild Kit
Or if you just want to upgrade the spring and add the o-ring, get this one:
BCM Extractor Spring Uprade Kit
Even if they don't make your gun a Wolf-eating AR, they will definitely improve your extractor to make your gun more reliable, too.
When you say it won't bring the next round up, you mean it won't FEED the next round? Does it do this with any magazines? Try a different magazine and see if it happens with that one, too.
Here's some info for you - sorry it's long, but informative.
Short stroking (Failure to cycle)
Short stroking on an AR rifle is when the bolt carrier group does not cycle far enough to the rear to be able to strip a round from the magazine and chamber the next round. After firing a round, you will notice that the bolt is closed on an empty chamber, thereby forcing you to manually cycle the rifle to feed the next round. This occurs when there is an issue with the gas system.
To see if you have a short stroking problem and not a “Failure to Feed” problem, please perform this test first (assuming your bolt catch and magazine is functioning properly). Insert one round into a magazine that you know to be performing well. Insert the magazine into the magazine well, pull the charging handle to the rear, then release the charging to load the round into the chamber. Safely discharge that round. After the round has been fired the bolt should automatically be locked to the rear by the bolt catch and the empty magazine. If the bolt is closed on the empty chamber, then it did not travel far enough to the rear to be held by the bolt catch, and your rifle has short stroked.
Short stroking is a symptom of a problem with the gas system in the rifle. There are 7 parts of the AR gas system. In the upper receiver group, you have the barrel (gas port), front sight base, and gas tube. In the bolt carrier group you have the gas key, the carrier, the bolt, and the gas rings.
Assuming the rifle was running correctly previously (in the exact configuration it is currently) The following is a basic trouble-shooting checklist.
1) Ammo issue ? Commercial ammo does not run as hot as Milspec ammo, and some lots of commercial ammo may not be strong enough to cycle the weapon. Try a completely different type of ammo. If the weapon works correctly then you may have some bad ammo. This is becoming a more common problem with some of the low cost imported ammo, especially with milspec sized gas ports.
2) Lube and clean the rifle. Pull out your GI manual and properly clean the weapon. As a note, a little bit of dirt and carbon should not stop the proper cycling of your rifle.
3) Check the carrier (gas) key. If the carrier key is loose, the gas system will not hold enough pressure to fully cycle the weapon. Try a different carrier that you know to be working in another properly functioning weapon. If the weapon runs well with the other carrier, you may have an issue with your carrier key. The carrier key bolts should not be loose. It should be staked from the factory. Check to make sure the bolts securing the key did not rotate past the staking. The bolts should be torqued to 35-40 in/lbs.
4) Check the gas rings. (or just go ahead and change them – they are very inexpensive) Mythbuster: The gaps in the gas rings do not need to be staggered. I know we were all taught otherwise in recruit training, and yes I know it says they must be staggered in the GI manual. But this is a myth. As a note; it will not hurt the system to stagger the gaps in the rings, but if this single change makes the difference – you need new gas rings ASAP. Inspect the rings, make sure there are 3 of them, and that they are installed properly. Disassemble the carrier group, insert the bolt into the carrier, and hold the carrier vertically in the air. The bolt should not fall out of the carrier.
5) Change the gas tube.
Failure to Feed
A failure to feed initially looks similar to a short stroking problem. A round is fired, a spent case is extracted and ejected, but the bolt is closed on an empty chamber. The difference with a failure to feed is that the bolt did travel far enough to the rear to chamber a round. But it did not, and it is now closed on an empty chamber. Generally speaking, this is most often caused by a defective magazine (specifically a problem with the feed lips). You might also notice 2 dents in the casing that is still loaded in the magazine from bolt over-ride. The solution is to replace the magazine. USGI magazines are class 9 expendable and should be replaced when defective.
Failure to Extract
This is signified by a spent casing stuck in the chamber, not fully extracted from the chamber, or one spent casing and one live round being stuck in the chamber. (Sometimes one spent casing and one live round stuck in the chamber is called a “double feed”, but a double feed is technically 2 live rounds stuck in the chamber)
Assuming the weapon ran correctly before and is in the same configuration, this is generally an indication of a failure of the extractor. The extractor is made up of the extractor, the extractor spring, and the extractor insert.
First remove the extractor and inspect it for crack, chips or deformations. If it has any, then replace the extractor. The commercial market also has seen some MIM extractors, so if your extractor has any MIM/casting type markings then replace it. The most common wear point for the extractor is in the spring. As these spring wear out they become weak and do not have the proper tension to extract correctly. Replace the extractor spring and extractor rubber insert. Be careful to order the correct part specific to your weapon. There are stronger extractor springs on the market made to address the increased pressure that the carbine systems run under. These are recommended. Also the rubber insert you replace should be replaced with a black rubber insert. The original M16A2s had a blue insert. A stronger black insert was introduced for the cycling of the M4 and is an improvement over the blue and is good to go in both rifles and carbines. Another fix is the D-fender rings, or O-rings found on the market. Install them over your extractor spring. This was a standard upgrade for the M4A1s done by Crane Industries for SOCOM.
Now assuming you have these issues on a new built weapon, then you may need to go past the above advise and look at some other areas.
Issues like this are most commonly involved on shorter barreled ARs. With a shorter barreled AR (generally 14.5”, 11.5”, 10.5”, 10.3”) you have about twice the pressure in the system with less dwell time (shorter than 14.5”) and a faster unlock time. One of the issues that needs to be addressed is slowing the cycle rate of these shorter barrels. This is done in part by the addition of a tungsten weight buffer. The M4 carbine runs a H-buffer and the M4A1 carbine runs an H2 buffers. There are also H3 buffers that see some use. Also stronger recoil springs will provide some aid. If all else fails you may need to look at gas port size and confirm that the chamber is not cut too tight.
Hope that helps.