Unbalance is a common issue that pilots will occur when take the BNF drones. Here is some methods you could troubleshoot the problem.
1) Maybe the battery is too close to the rear of the drone. You could insert it forward to change the barrycenter of the drone.
2) You could throttle up the drone leave the ground to ignore the influence of the airflow.
3) To check if the motors on the rear is defective or block by something, like the hair.
4) If you still could not balance the drone, take off the canopy and try again.