RE: Hitech 5645mg fails during installation
6.0V is fine...
Did you try another known working servo in the same location same time to verify the servo itself was the problem?
Charge the battery and try again. Did you check the battery voltage under a load, at least a 1/2 amp?
Cycle the servo connectors to ensure good contact.
Is a wye in line with this servo?
It's plausible the servo is defective if nothing else.