Another thing you can check is the main wing to the fuselage alignment. It should be 90 degrees to the fuselage when looking down and have equal angles when looking from the front or rear of the plane. DO NOT HAVE THE PLANE SITTING ON THE WHEELS when doing this, have the fuselage blocked up so the fuselage is 90 degrees to the ground - I would also recheck the horizontal and vertical stabs alignment. If the wing is off center, line it up and put some reference marks on the front and rear of the wing. If the main wing is off center it can cause this (one side of the wing has more lift than the other). The reason for blocking the fuselage is the landing gear/wheels may have a slight difference in height.

Have you checked the "differential" in the ailerons ? They need to have the same amount on each side. If the differential if different or they don't have any differential that can cause the problem. If there is no differential, you should put some differential in the ailerons, it will make your turns easier. Differential is where the ailerons go up more than they go down. You would put the push rods off center on the servo to do this and it is easiest to do if you use a wheel instead of a servo arm.