Just remember, when you're doing an inverted flat spin, you give RIGHT rudder, and flatten it out with RIGHT aileron. When doing a regular flat spin, you give LEFT rudder, and flatten it out with RIGHT aileron again.
I have and have had planes, where I've had to feather all but the rudder input to obtain a flat spin. If I gave full elevator, it would tumble out, if I gave full full aileron, it would roll out. Having the plane good and tail heavy and lots of throw on rudder is a good start. Also, feathering the throttle will some times make the flat spin better, spinning more around its CG.
DKjens