I suspect it is a trigger bar binding. I had the same thing happen on a gun I am currently building.
My set triggers work just fine before I put the lock bolt in and threaded it into the trigger plate. That little bit of tightness from the lock bolt pulled the trigger assembly in slightly and the front trigger bar was hanging on the front of the mortise. I blacked the trigger assembly, reinstalled it, tried to make it function(it wouldn't), pulled it back out and could clearly see where the front trigger bar was binding on the wood. A little chisel work and it worked just fine.