Not familiar with this weapon, I need to ask a few questions:
• Does it have a floating firing pin, which is kept away from a cartridge primer (until the hammer hits it) by a spring? Maybe the firing-pin tunnel, or the spring, is either dirty or packed with grease, which binds-up the firing pin. (This is the most likely culprit.)
• When you push on the firing pin, and its nose comes through its hole, is it smooth and rounded? Maybe the firing pin is broken, and won't reach all the way to the cartridge's primer. Maybe the rear end of the firing pin is broken, rather than its nose.
• Do the rounds which did not fire pass the "chamber-check" test? Maybe the rounds which did not fire are just a little short, or are crimped a little too deeply to meet headspace criteria. Compare head-to-case-mouth lengths: unfired samples of rounds which fired, to unfired rounds which did not.
There is a house in the historical part of Providence, RI, which bears on its gate the following signs:
"Attention: Chien Bizarre"
"Chien Fort Méchant et Peu Nourri"
"Oubliez Le Chien. Attention au Maître!"
Could your home in Houston use one or more of these?