The piston may be stuck. That hex "screw" isn't really a screw, but an adjuster that's geared to the piston. Turn it counter-clockwise to retract the piston. You might try wiggling the piston with channel locks (don't scratch the piston - use a shop towel to protect it) or tapping it with a mallet to see if it's cocked in the bore (boy, that sounds weird). Anyhow, hope that helps... also, be careful that you don't lose the adjuster - they're not really held in so they can fall out if the you turn the caliber on it's back.
Of course, it could be that the piston is already retracted all the way back into the caliper.