Frog,
It is surely the set-trigger mechanism, but probably not the outside screw on the bottom of the trigger plate. You would certainly have tried that first. There is another screw on the inside of the trigger plate that adjusts not engagement but spring tension. That screw WILL tend to back out after a lot of shooting. 'Happened to me the same way. Solution; disassemble, adjust, reassemble, try, repeat as neccessary. :roll: :wink: