Does this have a LIFEPO4 battery? If so, there's a Battery Management System (BMS--a computer board) on the battery that protects the system that may be shutting down when the throttle's full--many people may buy the appropriate Volt battery but do not realize that they need to look at the maximum continuous discharge amperage of the controller and make sure the LIFEPO4 has adequate discharge ratings. Otherwise the BMS will shut the system down when the throttle's opened up and the amperage peaks.

If the battery isn't LIFEPO4, there may be a controller or throttle issue. If you have a volt meter, check the reading on both.