One motor stops spinning when throttle is pushed down

Our motors seem to spin at the same speed when armed and when you lift the drone off the ground but the moment you bring the throttle down the one motor really slows down and sometimes shuts off causing the drone to crash. We recalibrated and checked motor direction and that did not help. Could this be a bad motor and need a replacement?

Did you perform a flight controller calibration or an ESC calibration? The two are different and it sounds like your ESC might not be calibrated.