I get this once in a while with my MD-12. I haven't *exactly* nailed it down, but I believe it has something to do with synchronization. That is, the camera shutter is not cocked and film is not advanced when you attach the motor drive. Out of sync means that the motor drive *remembers* that the last thing it did was advance the film and cock the shutter. So now, it won't do its job because it thinks the camera is already cocked.
When this happens to me, I manually cock the shutter so that the camera is in the state that the motor drive believes it should be in. The reverse is also possible. That is, the shutter is cocked when the motor drive state says it should not be. I know, this sounds like hocus pocus... but by fiddling with the camera with the drive disconnected, I can get it to work.
I would love to hear if you can get your drive to work by this kind of fiddling. That would prove I am not nuts.