Is the main spring strain screw tightened all the way in?
I encountered that once with a set of Pach's that would not go on a buddies gun, he had done the "classic" maneuver of backing out the screw to lighten the DA pull.
A few turns of the screwdriver arched the spring so the grip screw cleared.
update: LOL... just saw H Richard beat me to the punch by minutes... after looking more closely at the strain screw tip, it doesn't look like there is as much protruding past the threads as a normal one, I'm voting it's a shortened screw.
This how the spring should look: