Check that the battery pack is definitely plugged in its under the floor panel, lift out the battery and check.
Do you have any power to the dash display horn, lights etc? did it take a charge when you first plugged the charger in? or did the charger just show green straight away?
Sorry i’m a little confused, so the bike has ELECTRICAL power? it switches on and comes to life, lights and horn work, and it drives? but it lacks power in relation to its performance when ridden? am I reading this correctly?
Probably best to give E-Rider a call and talk through the problem with Ken.