Hello, I just purchased a BetaFPV Cetus(brushed) and I have been flying it for a while now and I put in a new battery. It was the original one. Then the back motor was not thrusting or speeding up. It arms but when I press up on the left stick the drone just flips upside down on the ground because just the front motors and the back left motors are speeding up. To confirm the back motor is arming. If I press all the way forward on the right stick and up on the left stick it will take off and go forward slightly but when I let go of the right stick it just starts flipping and falling because the front motors are working correctly and the back one is spinning slowly I checked both back motors for hair, carpet, ect. but there was nothing. I was just wondering if you knew how to fix this. Thanks! This is the brushed version of the Cetus. I have not had any magor crashes either.