Flat bottom wings are mainly used in slow and relatively light powered models.
A flat bottom wing often tends to "balloon" up into the sky, keeping climbing
when full throttle is applied or the model is overpowered.
You may start doing the adjustment you think is easiest to do (e.g. shim the trailing edge of the wing up).
Moving the CG slightly forward may also help but you'll get higher landing speed if it becomes too nose-heavy.