Sometimes there is a little overhang of metal along one edge of the chambers after the face is machined flat. When the ball is rammed in it gets shaved out of round or down to a smaller diameter that no longer grips the chamber wall. One way to get rid of "ball creep" (NOT a venereal disease) is to slightly chamfer (bevel) the edges of the mouth of each chamber. Euroarms does this at the factory (all 7 I have owned were/are chamfered). The Ubertis weren't. I have taken either a rounded stone or a countersink drill bit and put a tiny bevel in each chamber mouth. A shallow taper will swage each ball like the Euroarms Remingtons. A short bevel might still shave a ring. The opinion that just because a gun shaves a ring that all will be well is bogus as you have experienced. You could get a chain fire with loose fitting balls or at least a jamming of the cylinder when they migrate forward. I have slightly beveled 2 or 3 revolvers to cure thm of ball creep.
With over 15 perCUSSIN' revolvers, I've been called the Imelda Marcos of cap & ball.
SASS#3302 (Life), SASS Regulator, NRA (Life), DGB#129
Wolverton Mtn. Peacekeepers (WA), former Orygun Cowboy (Ranger, Posse from Hell)