These guns came from the factory with amazingly bad DA pulls, so it's common to find them with aftermarket spring kits installed. For whatever reason, that often results in timing and carry up issues, which can sometimes be a little tough to detect. The result is that when manually cocked the cylinder indexes correctly, but DA use sometimes results in the hammer snapping on a previously fired case.
To detect this, make sure the gun is unloaded and then slowly cycle the DA trigger while applying light pressure to the cylinder, to impede its progress. This might have to be done quite a few times. The cylinder should rotate in one direction throughout the trigger cycle. If at any point you feel a distinct "unlocking" and can then turn the cylinder in the wrong direction with your fingers, you will have found the problem.
In one case which I personally witnessed, even that test was not adequate, and the problem was diagnosed by watching the gun during live fire. Every once in a while the cylinder would unlock and turn just slightly "backwards" as a result of recoil. The next cycle of the trigger would then do nothing but return the cylinder to battery with the just-fired case under the hammer!
Unfortunately, the only guaranteed solution I have found is to return the gun to the factory. In some cases the problem can be solved by replacing all of the springs with full power aftermarket parts, but unless things have changed, even the heaviest aftermarket springs are not as strong as OEM parts, so sometimes still fail to solve the trouble.
