Which thumb screw are you having trouble with? The ones on the axis of rotation that adjust the rotating tension, or the one that screws down onto the rotating shaft (absolutely useless). if the later five minutes work with a file making a flat on the shaft would stop the rotation.
I haven't had the problem with hook size adjustment. Its been steady through a whole pack of hooks. The only thing thats thrown me is having the fully tightened position when the lever is parallel with the bent arm.