Electrical connections to motors
Significant and complete loss of power can occur due to poor electrical connection between the Motor Controller and the Motors themselves.
Vibration can cause the connectors to the motors to come loose, causing a loss of power.
Loose connections will heat up causing carbon deposits. This will cause even more loss of power.
The wires between the Motor Controller and the Motors are thick and made up of many small strands of wire bundled together. It is possible that the many wires in the cable can break without any visible damage.
Loss of power will only be detectable by the operator pressing the throttle without proper response from the motors, and may be confused with a dying battery or broken Motor Controller. There will be no indication from the LEDs. If the wires are heating up, the battery will drain as if a lot of power was being delivered to the motors, but in fact was turned to heat.
Check connections with a volt-meter. Go over all heavy wire electrical with a soldering iron. Make sure connectors make solid electrical connection to the motor, bend connector into shape with needle-nose-pliers if nessary, so that it is not loose.