The answer is, the nipple cone is slightly too large for the caps your using.
The solution is to remove the nipple and put it into the chuck on an electric drill. Try to clamp on the round body rather than on the threads.
Now, using a flat metal cutting flat file, place the drill on your left with the chuck pointing towards your right.
Slowly start the drill and place the flat file against the side of the nipples cone. Push the file forward once or twice and stop the drill.
Now, try the cap on the nipple. It should easily slip on until the priming compound rests on the end of the nipple.
Once this is done, the need to physically force the cap in place to make the gun fire on the first fall of the hammer fall will be gone.