You need to hone your chamber. Small imperfections in it allow the shotshell metal to flow into their grooves and get stuck.

The cheapest cure to the problem is to use a section of cleaning rod with a 20 gauge brush in it (if its a 12 ga). Wrap the brush in steel wool so it's thick enough to fit firmly in the gun's chamber. Mount the rod in a drill. Spray everything down with gun oil before and during the process of running the drill full speed in the chamber for about a minute (move the drill in and out a ways while you're at it), and repeat the process a couple times using finer grades of steel wool. When you're done, clean the barrel.

The chamber should be mirror-shiny and the gun should operate properly.