I noticed you said "factory nipples" That is the first thing I do on a new cap gun, change to Tresso nipples. I have yet to see any factory nipples that are not junk or don't fit the standard cap sizes. If I order a new cap gun, I order nipples for it and hopefully the nipples come at the same time or before the cap gun gets here. I've seen factory nipples of different lengths, to where I didn't need a micrometer to see the difference. And flash hole sizes all over the place. And soft, soft steel to where they batter down and erode quite easily. I use Tresso's and the fit a Remington #11 perfectly. I did install a set on a second gen 1860 Colt recently and had to grind a couple though off the ends as the hammer wouldn't go down far enough with a cap seated to reset the action and allow the hammer to be worked and cylinder turned. That was a first though.
Change your nipples, that will solve a lot of issues with a cap gun rite off the bat.
There may be issues with a light hammer spring, or with drag in the internals, but the fastest and most likely correct fix is decent nipples.
And I hammer seat all my cap guns. I've hear the safety argument, but here's the thing. If you hand seat you and you have a detonation you have the chamber out of battery and who know's where the ball is going to go, and your hands are mighty close to all that flame and ball going where ever. If you hammer seat and you light the cap off, the chamber is in battery and the ball goes down and out the barrel like it's supposed to. The hammer is the only thing designed to ignite and survive a cap detonation, and the barrel is the only thing designed for the ball to go down. Just make sure your pointed down range in a safe direction. I hook my thumb deep into the hammer when working the hammer/cycling the action, and have yet to have an accidental discharge that way.