Aw, Skeet... I am so proud. Seems like yesterday you had no clue. Now, you diagnose a cocker like an old hand.
1/8" allen for the lug, unless it is an unusual lug. Then, it will be 3/32". Could be the SON is too low, as well. Could also be that the sear solenoid has come unplugged from the board. Could be the mainspring is too short. Could be a small troll is wedged inbetween the receiver and the hammer. Don't see that often, though. If the hammer moves ok when the grip frame is removed, my money's on the lug being too deep.