I have somewhat over a dozen Uberti 51s, 60s and 61s, also 6 Remingtons and have yet to have one fail out of the box. They range from 30 years old to only a few. I shot two 1860 Armies at a CAS match last month that had never been fired before, they never missed a beat. (I do replace the nipples with Treso, however}.
Uberti has lightened the mainsprings some in the last few years, but if you use Remington #10 caps, and they are fully seated, they should not fail to fire.
Make sure nothing is holding the hammer back from a full stroke, make sure the wedge is driven in to where the hook of the spring is just at the edge of the slot on the right side, no tighter, no looser. Make sure the mainspring screw is tight, and there is no drag on the hammer fall. Often an Italian gun needs the bolt wing beveled to reset over the hammers cam smoothly. The lack of that can cause drag and slow the hammer fall. See a book on tuning Colt Single Actions.
Let us know how it goes.
