MK129 won't travel straight ahead

My assembled MK129 consistently circles to the right, rather than heading straight towards a light source.

I tried adjusting the trim. With RV2 turned to lowest sensitivity, RV3 to highest sensitivity, MK129 circles to the right just fine. RV2 set to highest sensitivity and RV3 set to lowest, MK129 struggles to circle to the left.

Assembly problem? Right motor problem?

Thanks!