I'm going to tell you something that you probably already know, but it's caused some problems for people who don't know, and that's this: All 12 gauge Remington Model 11's use 2 3/4 inch shells, measured when the shell is an empty hull. (Once it's crimped its shorter than 2 3/4 inches.) The guns may fire slightly longer shells but won't load or eject them cleanly (or at all).
Also, Model 11's can be adjusted to shoot light or heavy loads. This is what the previous poster was referring to when he mentioned friction rings. I've never had the problem myself, but I've heard that setting the guns friction rings for heavy loads but then shooting light loads in it will sometimes cause the problem you are experiencing; but, this is just one of a number of possibilities. However, if you are shooting the correct size shells, then this is the next thing I'd try to eliminate as a possible cause. Instructions on how to adjust for light or heavy loads are printed inside original Model 11 forearms.
Anything much more complicated than one of these two causes would send me to a gunsmith. Model 11's are legendary for toughness but they're not indestructable. If anythings broken, Bob's Gun Shop (in Arkansas) is a good place to find parts.