I have the exact same pistol as you do, down to the nipples.
The Uberti nipples are kinda-sorta famous for not being too well made.
You can either replace them with a set of 12X28 for the pistols (The back end is shorter than for a musket), or polish the heck out of them.
T polished my originals by chucking the threads
carefully with a bit of tape wrapped to protect in a power drill & buffing the heck out of them with scotchbrite pad strips.
The Ubertis work fine with CCI #10 caps now, but I also got a replacement set that takes #11 caps.