Occasionally the chambers in the cylinder are too rough (poorly finished). When the brass expands, when fired,! it will flow into the irregularities of the chamber walls. I suppose hotter ammo would expand more than lower pressure stuff.

My gunsmith has fixed that easily by polishing the chamber walls. He puts a very fine polishing compound on a shell casing, puts it in the chuck of his drill press and slowly rotates the treated shell casing in each chamber.

Sounds easy, but he's been a gunsmith to 40 years and makes lots of things look simple. I wouldn't try it myself.