We've been seeing sheet metal housings where the pin holes are a hair too close together. To test this you can put the hammer in the forward position and slowly pull the trigger and release it, if you feel any binding it's a good bet the housing is the culprit.
To correct this we remove a little material from the back of the firing pin block arm, where it meets the pocket in the sear. CZ-UB has also discovered this issue as we've seen several factory guns with hand fitting in the same spot. You can use a nail file and go slow, there's no shame in sanding a bit and testing it a few times until it's running as it should.

