I get handed several caplock guns with this problem each year.  A few of 'em I've repaired on the spot.  Unscrew the nipple and see if it's clear.  If it is, chuck the threaded end lightly in a battery drill motor.  With a nail file, ignition file, or the file on your Leatherman tool, dress the flared portion of the nipple, being sure to bevel (chamfer) the egde a little.  Put the nipple back in the gun.   
Go shooting, shove the cap fully onto the nipple for each shot.
Three Hawks