Widder has made a good suggestion! If that isn’t it….
Assuming two things in the lack of information presented so far:
1. This has never been a problem with this gun before.
2. You examined the firing pin and confirmed that the bushing is in place and the pin protrudes out as it should.
Confirm that the correct cylinder and the correct base pin is installed all the way in.
Also assuming this gun is actually a .38/.357.