hanging up in half-cock

[muffinman]

I recently purchased a new rifle musket which hangs up in half cock when the trigger is squeezed. If you pull hard on the trigger, it will drop all the way and pop the cap. Could this be something in the tumbler, or perhaps the sear? This was a defarbed musket, bought on-line. I think its an Armi sport.
Dave Courtenay, Cprl. 67th ny