Hi Pete,
Either what Peter suggested re the speed or the power supply hasnt enough juice to run the motors, check the rating and see what Amperage it is, ie 1A, 2Amp etc.
My 240V AC plug pack is a 2A one and it drives it fine, when I plugged in a 750ma one, same results as you described...
I run my scope mainly off a 12V powertank from supercheap with a straight 12V cigarette lighter plug that came with it. Perfectly fine with that as it has oodles of current.
Also, download the HEQ5 manual
here and have a read....honestly, reading the manual is a good thing....
Hope this helps
Cheers
Chris