John it is probably because the threads in the wood are starting to strip. If you do drill it and put a dowel like Bill is suggesting, I would buy a plug cutter that is made for making tenons or dowels. Use a piece of dense grained walnut and cut the plug long grain not end grain. End grain has more of a chance to split and strip. These plug cutters give you an almost exact size and 3/8" would be what I would and have used.