I have this same problem with one of my newer ARF's (Magic Extra 300L). I was able to do a blender a couple of times during the first 8 flights but can't seem to get it to go lately. I was thinking maybe the wind or the fuel level had something to do with it. It's possible that my CG was too far forward and I did the manuever at almost empty tank and made it work. I heard it's bad to do them at close to empty since the fuel will all gravitate away from the engine and either run lean or flame out.

I have the Chief Aircraft FunAir 3D 40 and that thing will flat spin inverted like a frisbee. Very fast rotation (2/sec), very slow decent. Maybe changing planes is causing me problems, I'm not sure.

Also the Magic I basically threw all the sticks at the same time. On the FunAir, I did it in stages since I was getting better. It could be that the Magic might need to have all the sticks thrown at once.

I'll keep trying though.

Some ideas for your questions:
-Increase all throws, especially rudder and elevator. Should be at least 45% deflectionl.
-Move CG aft little by little (1/2 inch at a time). If your CG is too far forward, the elevator throw will have less effect and not flatten out the spin enough.