The "best" answer would be that the firing pin hole is gummed up and the pin does not retract when you close the gun. A good cleaning may be the cure.
A more difficult problem would be the upper edge of the tip of the sear (usually the right) has become rounded. This image courtesy of David Williamson. Fixing this requires a very good smith with a small file
Let us know where you are and we might be able to recommend someone nearby.