Mine does the same thing after the first 6 shots. Gets dirty I suppose.... Using CCI #11s. Stock nipples on ROA. Anyway, what I did was make a short pushrod (a hardwood dowel with a ball on one end) that I could completely press the cap onto the nipple after placement. The little ball on end of the short rod is to 'comfort' the palm of my hand while pushing. That did the trick for me. Note that my hammer mainspring is quite strong and has never been messed with.
A clinger. When guns are outlawed, only outlaws will have guns. Single Action .45 Colt (Sometimes improperly referred to by its alias as the .45 'Long' Colt or .45LC). Don't leave home without it. Ok.... the .44Spec is growing on me ... but the .45 Colt is still king.