The only things I can think of are:

1. Your tail motor may be burning out.
2. There may be some binding that keeps the tail rotor from spinning freely.
3. Make sure the polarity is correct on both motor plugs.

A side note to make it easy, the tail motor, red and black wires, should have the red above the black, and the main motor, orange and black wires, should have the orange below the black. You may want to buy an extra tail motor anyways, they're cheap and they burn out all the time anyways, so you'll use it eventually even if it's not the problem. Good luck. I just got done giving my CP a major overhaul. Flew well until the battery died.