Just make sure your mag well surround on the trigger guard is level and not bent down this may explain when you tighten the screw down the first pressure disappears.
I think its meant to mean won't tighten any further if you cannot get it to bottom out and tighten up then the threads in the receiver might be cactus, I think the wood fore end has to be on I am sure others who have the finesse will be along in a second......

By the way welcome to milsurps have a Good Xmas & Happy New Year.