:blush: I thought there was some problem with the USB connector itself!
I had installed a MegaMoto shield on my Arduino Mega 2560, but It seemed I had to apply downward pressure to the USB plug that was inserted in the USB jack on the Mega 2560, to get my motor to turn!
I got to looking at the situation and I discovered that the ENABLE jumper pins protruding through the bottom of the MegaMoto pc board were in contact with the metal cover of the USB jack on the Mega 2560.
My quick-and-dirty fix was to super glue a thick square of non-conductive plastic sheet to the top of the USB jack on the Mega 2560
It seems good practice (in general) to use long threaded machine screws and some hex nuts to position shields safely above the main board, in order to avoid situations like this one.