Re: 1858 New Army pistol
caps do get larger. Caps are supposed to have enough spring to their metal that they grip the nipples. However, the metal in your caps may be to soft to stay gripped to the nipple. or the nipple cones are too smooth for the caps to grip, The angle of taper is quite critical and the Italians have a problem in that regard.
The alternative is new nipples. Alot of owners of Italian wheel guns find they have nipple-cap problems. (Me included) Seems a little odd, but a little corrosion on the nipple cones may help.
Some caps are hard brass and stay on just fine. As long as you don't mind using a knife or screw driver to scrape them off the nipples after firing.
(In this answer I assume you mean they won't stay before being fired. Alot of guns have problems with caps and fragments falling off after being fired.
You may have to suck it up and buy other caps, or a set of new aftermarket nipples. Treso brand are normally considered very good.
Cap and ball revs are alot of fun, once you get the frustrating bugs worked out.