Do you have the controller?... if so take the joysticks and move them together and down at the same time... this will cause the motors to start but not take off. (To take off move the left joystick up)... with the motors running look to see if anyone of the motors are spinning different... check that motor to see if damaged... you can also just watch it to see which way it flips over (it will most likely be the same) the motor it flips to is the problem... it may need to be replaced- which is an easy diy project...
Try to calibrate.
Try a new prop on the side its dipping.
When doing a throw start check if all motors is speeding up the same.
Mine started dipping at take off when one of the brushes started failing in one of the motors.
I had this problem while running the calibration procedure, and found that I had inverted the elices, if I noticed that there are two with markings and two without, the ones that have markings should be placed on the motors that have markings.
Attention the motors markings are very discreet.