Chances are the loading nozzle is broken. I had the exact same thing happen to my KSC G19. The part you're looking for is part #22 IIRC which coincides with the loading nozzle/cylinder. It's like a $10 part. Take out the innards and transfer them to the new loading nozzle and you'll be back up and running no problem.
The reason this happens is because the loading nozzle that feeds the BB's is plastic and eventually breaks leaving the loading nozzle to not load BB's into the chamber properly and as such doesn't allow it to fully return to battery.
Heres a guide to show you how it's done. Note: this is a guide on upgrading the float valve/rocket valve but the principles are generally the same,
http://www.airsoftforum.com/board/lo...p/t126819.html