I have a falcon 40 and have experienced this problem. I fixed mine by checking the rotor blade shaft. It has small plastic tubing, I believe, to keep the rotor blade engaged. Actually, I had to use one of the small rubber thingies that holds the canopy on to replace the lost plastic tubing. Every so often, things move on the tail rotor shaft and I have to reposition things to re-engage the tail rotor.

This might not be your problem, but then again it might.

Good luck!