And you can not see anything "odd" when it is parked at 12, sheeted respectively in and out?
It should be dead straight in both cases.
I would have a friend watch your kite, or even better fly your kite while you watch it closely.
Fly it down to hover a bit over the ground, and again sheet out and look closely, and sheet in and look closely.
Then fly it to the other side hovering, and watch again.
Do this close to the kite, and a bit away, try to find a difference.
If you do this a couple of times you should be able to see something odd
How long lines are you using, and high or low V?
WHEN exactly does it happen - could be a clue also.
Say if it was when jibing and riding towards the kite while pulling one side of the kite hard?