Sounds like you need more power if you are already tailheavy and at 50 degrees of elevator deflection. A good flat spin usually requires almost full power even on a heavily overpowered 3D plane. Try an inverted flat spin and see what you get. Most planes spin better inverted than upright.

Heres how you do it.
Get up 3 mistakes high
High rate elevator.
Full Down elevator, Full Left aileron, Full right rudder and about 3/4 to full power at the same time.
When the plane starts to spin let the ailerons come back to center and adjust as needed to flatten it out, you may actually end up with a little right aileron depending on how the plane acts. Everything else stays at full throw, maybe adjust the throttle some to clean it up.

What plane, engine, how much does it weigh??