Adafruit motorshield

I am having issues with my power supply flashing on and off when I try and hook the v2 motorshield to an Arduino M0 pro. The power led in the shield flashes on and off as well as the board. I am using a 12v 3A power supply.

With literally nothing connected except the jumper on the shield and the dc power jack on the arduino every time I try and power both at the same time the power supply literally turns on and off like it’s shorting out.

When I remove the shield the Arduino powers up fine with the power jack.

If I remove the jumper and bring power to the shield the shield powers up ok but as soon as I go to power the Arduino with a line to vin I get the turning on and off of the power supply.

I also tried powering the Arduino with a usb while the shield was getting power from the power terminals not jumped on the vin jumper. Still I get the same thing.

What is going on here? It is as if they are shorting out on each other?