It may because the robot is not pumping water.
To see if the pump motor is working – see if water is being pushed out of the top of the robot when in the pool.
Next, check if the propeller is spinning?
It also can be due to the propeller being clogged with debris (hair, string, etc.) Remove the outlet top (make sure the machine is turned off and has no power running to it) and see if the propeller can move freely by rotating it with your finger or a pencil.
Are there obstructions in the pool?
Remove any obstructions from the pool walls (ladders) if necessary. The robot could be hitting the obstructions and falling backwards.
Clean the filter bag.
There is a chance that the filter bag may be clogged. Remove the filter bag and clean it. Then try running the robot without the filter bag and see if the robot operates normally.
Check the handle for buoyancy
Place the robot into the pool, totally submerged. Then, remove the robot from the pool and shake the handle. The handle should not have water sloshing inside it. If it does, the handle will need to be replaced.