As it usually is, the key to finding a solution is for YOU to provide more information.
First, how do you KNOW it has a full charge, and do you have a voltmeter?
Next, when it shuts down, can you wait a few seconds, and turn it back on?
What do you do in order to use it again? If you charge it, for how long?
Look at ALL the lights, display, indicators of power. Do they ALL go dark, or do any of them still work?
Does the battery itself have a power led, and or a switch to activate same, and if so, what do these show?
ALL ebikes will shutdown, after a period of time or a number of miles. How long and how far do YOU get before it shuts down?
Is any error message displayed before shutdown?
When it "shuts down", describe EXACTLY what happens.
Lastly, disconnect your ebrake sensing wires and leave them off until the problem is resolved.