I have pulled the valve completely and have water coming out full force. When I replace the valve on turn the water on there is no flow to the shower head. Does the mixer valve in the back need to be replaced? I've replaced valves and O-rings before, but have never had to replace the mixer valve.

Make sure all the holes are open and not plugged with debris. Moen will send you a new mixer valve free if all else fails.

You may have clogged filter screen at shower head. Remove shower head and run water without shower head - just through shower arm. If you have water flow, look inside the shower head inlet There is a screen, probably full of debris. Clean it under running water, reinstall.

Hope that helps

Milo