sounds like your locking screw is stripped and you need to get the stud loose before the smaller screw will turn. if you dare, you can put an
allen wrench in the stud that isn't moving and give the allen wrench a few taps with a hammer or something. enough to jar it loose. tap it so you are going counter-clockwise and not tightening more. then once that budges, you can loosen the small one and adjust it if it's stripped out.
if that doesn't work, spray a littlw WD40 on that stuck stud and try again.
hope this helps.
JOhn