Electric over hydraulic brakes

you should be able to lock up all wheels. something is very wrong. guessing from here is just that, a guess. I would start by checking fluid in pump and then bleeding brakes. Is the pump running fully or is it running on lower voltage than it should? Could have undersized wire and a big voltage drop? Controller could be bad. How about going to a deserted place and have someone in the bed pull the breakaway switch at a low speed and see if tires lock up. Are pads free and able to move properly? Are pads good or totally worn? All good questions with answers only available from you.