Are you absolutely sure you put the batteries the right way? First time I did that I thought it was obvious which way the batteries go and then suspected everything else for the damned thing not working, and even went to a repair shop to let them sort it out. Boy, did I leave that shop ashamed!
Other than that, I can't help you as I never test fired it after it "miraculously" worked again and I don't own it any more.

Good luck. MD-12 is a nice winder with the best grip in the history of universe.