This issue is most often caused by one or both of the following.
Misaligned Rear Tire(s)
If your vehicle is behaving abnormally, check if either of the rear tires are coming off or are misaligned on the wheel hub.
Rear Axle Obstruction or Gears Damaged
Sometimes hairs, fibers, and other debris get caught between the rear axle and the wheel hub. This debris can hinder, slow, or completely stop a wheel from turning.
If you see any debris around or caught on the rear axles, pull it out using your fingers or a pair of tweezers.
If the wheels and axles look clean, the gears inside of the vehicle could be damaged.
Sometimes you can detect gear problem by using your fingers to slowly spin each of the rear wheels. While it helps to compare a problem vehicle to one that does not have any issues, manually rolling the wheels of a vehicle with gear problem don't roll smoothly. There's a drag, and rolling the wheels feels rough or bumpy.
If gear issues seem to be the cause of your issue, please contact Anki Customer Care.