i have been having a similar problem with my brand new pietta 1858 from cabela's. i have the 1858 with an included spare cylinder. i have zero experience with black powder guns although i have some other shooting experience.
with one cylinder everything seems fine. cylinder revolves, locks, unlocks when it should. hammer cocks and half cocks. loading lever seems fine. it all looks pretty sweet too. i haven't had a chance to shoot it yet with all the lousy weather we've been having.
now the other cylinder is weird. almost everything thing seems ok except the hammer will not stay at full cock. cylinder revolves ok and locks when i pull the hammer all the way (or seems like all the way) back. but the hammer won't stay at full cock and sinks back to half cock.
funny thing is the bad cylinder is the one that came installed. the good cylinder is the included spare. the only difference i can see is the good cylinder has some proof marks, bad cylinder has no markings at all.
if there is any easy fix i wouldn't mind doing that. i figure it's all part of the adventure. at least i can play with the good cylinder until i work up the courage to do some fixes. if it's the hand length, any other tests to try? what would i need to do to fix it?