I had the same problem but fixed it in two ways. That's cuzz there's kinda two problems going on.
I used a dremel to open up the cup at the front of the hammer face so I had good clearance. That helped but I was still getting failure to fire now and then, especially after a few shots that fouled the nipple.
Now, after seating a cap on the nipple I lower the hammer and push on it to seat the cap all the way down on the nipple. Even on a clean nipple, the cap always moves down another 1/16" or so.
I learned that trick from someone here on the forum. In my eyes it seems like the standard Lyman nipple has a little too much taper on it. I keep intending to chuck one in a drill and turn a tiny bit of the taper out of it, but never remember. I'd rather have a nipple slightly too small and crimp the caps to make them stay on, so even if take out a little too much taper, I think I'll be ahead.
All short term fixes rather than bending the hammer. But I think that even after you bend the hammer you're either going to have to work at seating the cap or changing/turning down a nipple to get 100% reliability, especially after the nipple fouls. Those caps just don't seat all the way down, so there's always some give when the hammer hits.