[Fingers McGee]

First off, who is the manufacturer & is it a steel or brass frame & how old is it?

Did the caps fire the second time you hit them?
Do the caps seem to be harder to install?
Is there an impression of the nipple in the face of the hammer. If so, how deep is it?
Is there an old cap pressed against the frame under the hammer?
Does the hammer or frame show signs of hitting.

There is always some play in the cylinder, otherwise it wouldn't turn.

It could be any number of things in addition to a stretched arbor or frame.

Hammer could be hitting the nipples, mushrooming them so the caps dont fit right and wont ignite - Had an 1851 Navy that ruined two sets of nipples before I figured it out.

Could be a burr on the frame (or an old cap fragment) not letting the hammer hit the caps - Had a Pietta '51 Navy that had a high spot on the frame where the arbor was staked. It wouldn't reliably pop all the caps until I filed the high spot down.

I've got an old Uberti '51 Navy that the arbor is loose on and the gun rattle like crazy; but it still fires every time I pull the trigger.