What do you mean all the wires are 12v+? What are these wires(going to the M4, ATX plug, power plug?) and how did you determin that?
What are you using to power the M4? if you are bench testing and using the 12V power from the AC-DC PSU make sure there is enough amperage on the 12V rail. I had the same issue with my m4 and a simple switch to using a car battery at 13V and a higher amperage did the trick