To start with real basics, do you understand how e bikes work Sean (apologies if im stating the obvious)
In any assist mode, the battery will supply power to the motor...up to 15.5 mph... and not beyond. Thats a legal requirement.
So you pedal, accelerate up to 15.5 then it cuts out and you pedal under your own steam. As (and if) you lose momentum and slip below
15.5, the motor will kick back in....and so on and so on.
If thats not the reason, something might be wrong but certainly my Crossfire, my wifes Motus....and i suspect most if not all ebikes come with a nearly fully charged battery so id be surprised if its a flat battery. I assume it is a new bike Sean ?