crud in the works woiuld be my guess. If you know how to do it, and have the proper screw drivers, pop the side plate off and see if there is a bit of primer or some such in there. Blow the works out with solvent, lube the pivot and slide points and see if the problem goes away. Make sure the integral lock hasn't shifted part on.....i haven't seen it happen , but i suppose it could.
If you haven't taken the side plate off a smith before, either go to a good smith, or study up on how to do it without buggering the screws and the metal to metal fit, there are several good vid's on youtube.