If I understand you right, you say you can move the tail rotor to max neg and pos pitch with the servo & linkage, but while your running, you don't get enough thrust to rotate clockwise... just enough to stop the couterclockwise torque rotation...

If that is correct, then I'd guess that:
1) You're not getting enough engine RPMs to make the tail rotor effective enough to overcome the torque rotation.
2) Something's wrong with your tail rotor blades... installed backwards, too small, etc.
3) Somethings wrong with your tail-rotor drive train... belt slipping, etc.

If all that checks out, here's a remote possibility...

I've read that if the tail rotor rotates down with the downwash of the main blades, you can sometimes have an ineffective tail-rotor thrust (due to the decreased "lift" on that tail blade). It's better to have the tail rotor rotate up into the downwash of the main blades (increases the "lift" on that tail blade). By reversing the blades, belt and servo control, it seems you could swap between these two modes of operation, for better or worse, (this also changes it from pusher to a puller tail rotor, or vice versa). I believe it's possible for your kit to be designed with the tail rotor rotating up into the downwash, yet reverse things the right way during assembly to make it rotate down, decreasing the tail rotor effectiveness... not real likely, but possible, I think!