Check the carbon drive shaft rod keyed end that has a small gear on the end which meshes with the main gear. On some original driveshafts this keyed end goes limp and the small gear attached to it will not be performing its job. Newer replacement carbon driveshafts are now being manufactured with a more beefier keyed end which is less likey to become damaged from a hard landing or main rotor boom strike. I have had this problem as you and changing the driveshaft fixed the problem. You might be able to fix the keyed end yourself by wicking the keyed end with CA and then let it dry completely before attaching the small gear. Best to order a new one though. The other thing is to check the geometry of your tail servo linkage that it is 90 degrees to the shaft at neutral trim both ends of linkage. Lastly, check the Tail Rotor hub setscrews that both are tight on the tail shaft. After everything is working again, recommend you take out one setscrew at a time and locktite. Alot of Falcon users have replaced all the Falcon 3D setscrews in the main shaft and the tail hub with 3 x 3 MM stainless setscrews. The chinese stock setscrews are prone to failure. You can get the stainless setscrews from RCSCREWS.COM. Come join us on the Falcon 3D thread here on RC Universe. Many users and many posts to help you.
Ralph