Check your base-pin latch screw to see if its backed off. If it loosens during shooting, it loses its grip. If this is O.K. Check the base pin latcth by removing it from the frame. It should be square shouldered, if its battered from recoil, it will allow the base pin to slip. Replacing this is a simple and in-expensive fix.
If all is well as far as battered parts, your base-pin may be undersized slightly. I grip the end where it enters the frame with a pair of pliers to throw up a burr at this point. This slight upset increases the diameter slightly for a more snug fit.