Assuming your CG is right, and you have enough speed through the turns... (sounds like you fly too slow...)

First I would have checked the wings for warps. Maybe they are twisted so there is some wash-in?
Do the elevator require much trimming? If so the decalages could be wrong. Measure using an incidense-meter.
Else: use differential throws on the ailerons. Usually 100% up and 20-40% down movement...
In addition to speed; use the rudder through the turns! This is important.