You say you have "ensured the cap is seated" and I take that to mean the cap goes almost all the way on the nipple freely and only the last tiny little bit does it require pushing it on.
IMO, that's the way the cap should fit.
If it goes about half way on and then stops and your shoving it further on until you can shove no more, then I must agree with the others. Your caps are not seating properly.
Different cap companys may provide caps that are just a little larger than the ones your using. If you can't find other brands to try, you can machine the nipple.
No, I am not expecting you to go out and buy a $1500 lathe. If you have a electric hand drill with a 3/8 inch chuck and a fine toothed flat file, your in business!
Remove the nipple from the gun and place it into the drills chuck with the coned "cap" end out.
You are going to "machine" this cone down just a little bit.
It won't take much to make a difference.
It is best if the chuck jaws are grabbing the outside body of the nipple, not the threads.
If you don't have a drill with a 3/8 inch chuck and you must grab the threads, do not overtighten the jaws. They should be just tight enough to prevent the nipple from slipping or coming out of the chuck. You won't be applying much pressure.
Now with the drill in forward position (if it's a reversable drill) and the chuck pointed to your right, lay the file on the top side of the cone of the nipple and start the drill.
Apply a little pressure to the file and push it forward once.
Now try your cap to check the fit.
If it still isn't seating fully, do the drill and file thing again.
After a few of these "adjustments" the cap should fit fairly freely almost all the way down to the seated position.
That should fix your problem.
Also, I have found that some locks are rather sensitive to proper lubrication.
It might be a good idea to remove the lock and oil the sear pivot, and the tumbler/hammer shaft using a light machine oil or a good gun oil.
If your lock has a "fly" in it (it's on the tumbler and it's really little) don't use heavy oils on it.