It could be that the adaptor (if it's also a regulator like mine) does some form of current limiting. Check what the adaptor says regarding current delivery. If it's delivering 14.25V at no load, this may by dropping a lot as the empeg powers up and draws current, so see if you can measure the voltage output during boot.

You need to be able to deliver about 1A at boot to get the drives spun up.

One of the few remaining Mk1 owners... #00015