First, please check and confirm:
- No hair or debris around the motors/bases.
- Motor connectors are fully seated at the board.
- All props have no cracked hubs and are securely mounted.

Next, to isolate whether it is a motor or ESC issue:
- Swap the problem motor with the one diagonally across from it. If your ESC's use motor connectors, please be careful to just pull on the top lip of the motor connector. You may also want to try a better screwdriver than the included one for the motor screws. 
- See if the problem follows the motor or stays in that corner of the drone.

*** Please note: If there is something jamming the propeller (i.e. twig, branch, tall grass, etc.) and the motors are forced to try spinning, you can burn out a motor or ESC. This is particularly prevalent when "crash flip"/"turtle" mode is used when it is not save to do so. (See the "Use with Care" section here .) Resultant damages are not covered under warranty, as such misuse/error is not a manufacturing defect.