There are three things that tend to flatten the spin. Number one is the increased drag on the inside wing when you apply opposite aileron... this makes it spin faster, which in turn makes it spin flatter. Number two is gyroscopic reaction forces from the prop, in an upright spin to the left adding power will help flatten the spin. And the third is actually down elevator in an upright flat spin... once you have it pretty flat with opposite aileron and power pushing down elevator stalls the tail and blocks some of the rudder... the result is that the nose actually rises rather then falling with the application of the down elevator. To recover power off, ailerons neutral, opposite rudder, neutral elevator.... if that doesn't work add power while holding opposite rudder and full down elevator. I've had at least two models spin to the ground doing flat spins, sometimes you just don't have enough rudder to recover if the cg is too far aft.

Ty