This is more of a techroom/newbie thread, but anyway. Stick your finger into the breach via the feedneck or (having unscrewed the barrel) the front of the gun. You should feel the detents poking into the breech. If they feel frayed or cracked you need to replace them.
This is how you do that: Take the grips off and remove the battery, then unscrew the frame screws (and the one above the inline reg just under where the barrel screws in). Gently lift the body upwards and take out the front banjo (thing with a hose coming out of it) so you can lift the body off completely (the board and solenoid will slide out upwards with it). Now, unscrew the other two banjos and unplug the eyes (the little white plug behind the middle banjo). With the board now separated, you can slide off the sleeve from the body. The eyes are the little u-shaped chip (which just slides out for cleaning), and just in front of them is where the detents are. pull 'em out and stick new ones in. Reverse process