like said above if the adjustment screws are old they will be worn in one spot. Kind of like a step on the surface and when you adjust them if you are just on the corner of said step it will beat it down pretty fast. the step I am talking about would look something like a quarter moon stamped/worn into the face of the screw.

hope that made some sort of sence.
I just used a sharpening stone to round mine back down.