I am no gunsmith, but it sounds like the firing pin is not striking as hard as it should. If you're getting dimples on the primer and a second strike mostly fires, that "could" be the culprit? I am always leery when when the trigger pull has been reduced. Sometimes it takes a harder pull to set off the round.
Remember, the 60 is not a target gun, but rather a self defense firearm.
Just a question.....on a dimpled primer that did not go off on the first pull, how does the dimple compare to primers that did go off on the first pull? Is the indent less? The same? That indent can tell a lot IMHO.