Here's a question. Does the model use a semi-symetrical airfoil? I had a student years ago build a model with the same complaint. It had a semi-symetrical wing and he built one of the wing panels upside down. You couldn't see it looking at the airplane but when we cut the wingtip off you could see that on the one wing panel the airfoil was upside down. If this is the case you will need to build a new wing.