Check to see that the tip and the mast at the partners are both centered. By that I mean centered in the boat, not necessarily in the hole through the deck which itself might not be centered. Even all appears equal, it might be that you'll need to block the mast on one side of partners in order to equalize the tension on the uppers. The North tuning talks about getting "mast and the keel on the same vertical plane" something to try next time the boat is out of the water.
