You'll prolly see the best results in a flat spin using full throttle. I typically go neutral aileron going into the spin, just full elevator and full rudder and bring up full throttle. Then keep wings level with aileron.

But the best way to get a plane into a flat spin that doesn't do it well naturally is from a blender. This time your nose straight down, half throttle, left aileron. Then when you get a little speed quickly go left full rudder, full elevator, neutral aileron, full throttle. Then balance the wings again with aileron. This method gets the tail whipping with centrifugal force. If your plane won't flat spin from a blender, then there's no hope for it.