Crud is often the root of feeding problems.
Beyond that, and with regard to M-12s, there are a number of possible causes. M-12s are, essentially, controlled round feed. So extractor hook wear or loss of extractor tension, on either side, can misdirect the shell. The LH extractor is the weaker sister, so shell noses might tilt toward the right. Sometimes a change of ammo fixes things, at least for a time.
A poorly timed/binding shellplate can cause misfeeds. A broken firing pin can present as feeding trouble.
If your M-12 is an original 28 there may be nothing wrong at all. Kinda normal for the breed.