I had the exact same problem last year.  2 out of the first 3 towers I was moving to a new set of bars stripped.  I didn't even bother to try the fourth.  Very discouraging!

I called Yakima to see if they would honor their "satisfied for life" guarantee.  No dice.  They said the warrantee had run out the previous year (I guess I lived too long!).  Anyhow, after some more complaining, I found out that they had completely redesigned the Q-towers, most likely because of this very problem.  The bolt heads on the original design were made from steel that was just way too soft.

Rather than turning a cam like the old design, the bolts in the new towers tighten up a clamp.  It puts much less stress on the bolts, and it looks like the new bolts are standard (i.e. harder) bolts.

Yakima graciously sold me two sets of the new towers for the price of repair parts for the old towers.  This was after I pointed out that I really didn't want to buy replacements that were also a defective design.