Check the relative angles between the wing's angle of attack. On all the bipes I've had (at least 15 including Phaeton 90's, Phaeton 40's, 1/4 scale Moth, 7 Aeromasters etc.) all flew best when the upper wing was about minus 1.5 degrees with respect to the lower wing. Just a degree will make a huge difference in how well they fly. Of course this all assumes that you have the CG properly set.