Not enough info to determine the problem.
Is the gun failing to feed a shell from the magazine, failing to feed one into the chamber, or is the shell getting jammed during the feed cycle?
The Marlin carrier seldom ever wears out or gives any trouble other than it and the entire action getting fouled. A good first step is to field strip the gun and give it a good cleaning and fresh lube.
Don't disassemble anything you don't have to. With the receivers separated and the bolt out, you can get to most everything for cleaning.
Here's a link to a current factory manual. Use it to disassemble and reassemble the gun.
NOTE: When preparing to separate or reassemble the receivers, make sure the hammer is FULLY COCKED to prevent bolt and receiver damage.
http://stevespages.com/pdf/marlin_39a39m.pdf
Here's an exploded parts view:
http://www.stevespages.com/ipb-marlin-39a.html
Note that this is for an older rifle. Later, the carrier was changed slightly with a spring loaded button instead of the flat rocker.
With the receivers apart, you can look at the right side with the carrier and operate the lever while watching operation.
Things to look for that can cause failures to feed:
Check the shell release in the left receiver...Part number 24.
Check to make sure it's not broken, weak, or impacted with grit and won't release a shell out of the magazine.
When the bolt closes the pointed triangular part on the bottom presses the release into the receiver and releases one shell. If the release is dirty or weak, it may not release a shell properly.
Make sure the magazine tube isn't gummed up and the spring has enough tension to push shells into the receiver.
The actual carrier almost never gives any problems unless the gun has been tampered with, or is so badly worn the lever can't raise the carrier.
This is rare.
