Rust may have started to build and bind the slide action bars. Like others have said, take the trigger group out. Then some spray wd40 on the slide bars as you slowly move the fore-end back and forth allowing the wd40 to work its way down and in. I think if it is rust, it's in this area versus the chamber. Hopefully it isn't too far gone and that will be all you need to do. If that doesn't help then I'd take it to gunsmith and have him completely dismantle the action and slide bars to get a better look at what you're dealing with and go from there. Good luck.