Please help I've got a problem that seams unique at least to me.

My muzzle loader only fires the cap every other time.
Unfortunately I missed my buck before I figured out the timing.
It seams that if it is cocked for some time ( haven't gotten a time frame on this yet) if fails to fire the cap, when I work the bolt it will fire ( the same cap), or when I put on a new cap (working the bolt it fires fine). Does this sound like spring fatigue. Any help on where to start would be greatly appreciated.
Also entered in muzzleloader board.

could be a weak spring or a deformed nipple

A weakened/fatigued spring will not regain it's strength. As much as the evidence points to the spring it's not the focus I'd take.