My standard break-in round for any rimfire are CCI Minimags.
Bulk packs are for the most part, Factory Seconds...which may or may not be within proper specs.
Ruger's are made with loose tolerances so that they will eat ANYTHING...much like an AK.
Marlins are made like AR's...picky & finicky until broken in well.
First question, serial number...is it all numerical or does it start with MM or MR?
The MM & MR ones are from the new factories. Hate to say it, but the all-number CT ones from 2005-2010 ain't no better.
QC at New Haven was almost non-existant from 2005-2010...and things got worse during those years.
The QC issues at Marlin's KY plant are being nipped as they are reported...
the guy who is heading up that branch is VERY highly dedicated to getting the guns made right.
They will eventually be producing the Marlins to the standard that we know and love...meanwhile, there are growing pains as the new guys learn.
Second...oil in the chamber will cause binding as heat, pressure, & oil combine to make Glue.
Make sure the chamber is totally clear of oil/lube/etc.
While yer in there, after swabbing with many dry patches, take a Q-tip and see if you have a Burr in the chamber.
Third, work the lever with FORCE & Speed...no weak sister-ing it.
Work it like yer in a John Wayne Movie, with Authority!
If that solves it, great...
If not, have another person familiar with Lever-actions look it over...
If they can't figure it out, send it back to Marlin as something is likely out of alignment.
Don't feel bad about sending a rifle back to Marlin if need be...two new plants mean a ton of new workers who need to learn.
As with any new MFR'ing, when you are working with high volumes of metal gear, things happen.
Why do you think the first year of any new car tends to suck?? New assembly lines mean new folks trying to figure things out.
With time, it'll become a seamless operation...just like Toyota's US plant
That being said, prepare for your phone call to customer service...write down the issues on a pad, sit it in front of you with a pen,
get the gun and sit it next to you, THEN make the call. Be nice, but firm about getting the issue resolved.
Be aware it may take some time, so pick a day when you have nothing to do and call Early.
Be aware that some of the phone persons know Bupkiss about guns, and some are experienced.
The experienced ones will probably ask you a few questions about function and get you to try a few things which may/may not fix it.
If you feel that the Rep isn't getting you what you need, Ask for a Supervisor.
This will take more time, but they have more leeway to resolve issues.
I wish I had a few quick answers, like I usually do for the semi-auto & bolt actions, but I have yet to work on a Lever-action,
other than to field-strip & clean the two big bore Marlins I own. I gotta get a 39-series rifle one of these days...
Marlin Owners and Rimfire Central have HUGE Marlin info/repair sections which may resolve your problem a lot quicker.
Chances are they've seen it and answered it about 100 times...