If the thumb throttle is not bouncing back, you can check if there is enough space between the throttle and the handle grip. If there is not, you should adjust the position of the thumb throttle or the handle grip so that there is a proper gap between them.

Also, check if the thumb throttle set screws are loose enough. If they are too tight, the thumb throttle can easily get stuck. 

If these actions still don’t resolve the problem, please contact the after-sales service for repair.