I hope you are talking about a reproduction.

Are you live firing or firing just powder charges?
What brand caps are you using?
What brand and size powder are you using?
Is the barrel absolutly clean with no oil inside.
What do you use for cleaning the bore after firing?

Misfires may be caused by a number of things.

The hole in the cone may be a little too small / fouled or the cone may need to be removed and the touch hole to the breech cleaned out. Sometimes there may be some raised metal in the breech area preventing the spark from reaching the powder or providing an area for fouling to build up. Oil residue in the barrel may cause a layer of oily powder to build up over the touch hole.

In many cases a through cleaning will correct the problem.