I have had a similar problem with a Ruger 77/50. If I place the #11 cap on the nipple with my fingers, the first pull of the trigger seats the cap, the second pull of the trigger fires it off. If I place the #11 cap firmly on the nipple with a capper, the first pull of the trigger will make it go off every time.

The caps you are having a problem with are stamped #11 versus #10?