Boardrack loose/slips?

My boards racks have always slipped downward when hitting heavy waves and I just can't keep them in the up position. I have tried rubber inner tubes, double sticky tape? The hex bolts are so tight that they are just about to be stripped out, but I just can't get them tight enough to quite slipping. They are Mako/boardracks with aluminium c-style clamps with plastic spacer and hex bolts. I'm going to shoot the off my tower if you can't help me. Thanks. Jet

Mine slip a little when I rotate them into the boat with a board on them (I think its just the added leverage.) I am planning on removing the clamp, drilling a small hole through the tower. On one side of the tower I will drill a larger hole to allow me to run a screw from the inside of the tower into the plastic spacer and clamp. Then I will put the other half of the clamp back on, over the hole (obviously). I figure adding this will keep the clamp from rotating.