If the wood has dried or shrunken for some reason, the top screw will rotate too far and be to the right of the index point. It can be caused by having the stock off and retightened to hard, thereby compressing the wood. I have fitted shims to correct this if the amount is slight. Often though, it can't be very thick or it will index at a point above the surface of the tang. It is generally caused by a wood problem and not the screw or head seat wearing. If it won't turn far enough to index, it is often due to interference in the screw head seat, under the top tang or interference in the action inletting.
