Mamba Micro X won't run in reverse
 

I recently installed a Mamba Micro X onto my Lossi Mini8 (the buggy, not the truggy). The motor runs great... ish. Forward is forward and runs perfect. But reverse won't function. When I look at the ESC, i see that it is changing LED color to indicate that it's recieving the reverse command, and it is sending the command to the motor to turn in reverse, but the motor is not turning in reverse. It doesn't turn slowly, or sputter. Or even make any noise to indicate it's trying to go in reverse. It just sits there. I've recalibrated the ESC a few times to no avail.
This is not the motor running in forward instead of reverse. The motor runs the proper direction when I go forward. But it just doesn't run in reverse when I move the throttle to reverse. From a standstill or from a forward motion. Never seen this happen.