Things that I look for when my rifle starts to do this. It has happened a few times, so now I check it more often..
Check your gas rings. Put the bolt in the carrier and turn facing down. If the bolt slides it is time to replace the rings. I change them when I can easily pull out the bolt.
Check your gas key and make sure it is tight on the carrier. Also the gas tube end and the carrier key do wear some, so it may be time for at least a new gas tube. I used to measure mine and have them handy, but I don't currently have them.
As was stated, check to make sure that you are using enough lubricant on the carrier and in the bolt. For the bolt, it needs lube and doesn't perform the best when there is little/no lubrication in it. It is a piston inside a cylinder.. lube it.
I have never seen a plugged gas tube except for one with a pipe cleaner stuck in it...