Greetings Monsieur Northcutt -
I had the same issue with my IAB 1863. The first thing I did was replace the #10 cap nipple with a musket cap nipple. That worked much better, as the musket caps have considerably more pop. However I began tinkering with the breechblock, desiring to make the famous "O-ring" modification which tightens up the gas-plate/ breech face gap considerably. Whilst mucking about with the breechblock I finally decided to completely strip it down. I discovered that channel from the nipple to the breech-block hole was blocked with scrap metal. After several broken drill bits and lots of careful work with dremel diamond burrs I got it all cleared away. The Sharps now ignites pyrodex regularly, which it would not do before.
I hope this helps -
your humble servant
Prof Marvel