Is the tail motor turning? These motors are susceptible to being damaged internally if the shaft holding the tail rotors takes a hard hit or if you crash and don't cut power in time. Had the one on my 120SR do this, basically seized the motor and had to replace it.
Another thing to look at are the delicate wires coming off the tail motor before it gets to any connector or wires with extra shielding. These are more or less exposed on both the 120SR and the nano cpx. On my 120SR I did enough damage to these that they frayed and I started getting under powered performance from the tail, and then it would just quit in mid air or while still on the ground. Now I cut a small piece of heat shrink tube to go over the end of the motor cage and shrink it down, to help protect those delicate wires. This provides some much needed protection for them and haven't had a fraying related failure since, despite some mammoth crashes.