Stepper sings, but wont walk....

Hey all,
I have been trying to repair a Bridgeport Boss that has a stepper
which when told to move via the stock control "sings" and will sort
of vibrate but wont actually turn. I've replaced every card in the
machine with those which I pulled from my retrofit machine (and
which are known to work), and the result is the same. I checked the
motor itself by switching it to a different axis circuit and the
stepper works fine.
So, I'm wondering what else besides the cards (drives, and stock
CNC) that could cause such a condition? I'll probably just retrofit
this one too, but it is making me quite curious. All that seems to
be left is cap's and wires.

Thanks

Sean