Jak-
I'll give you my input although this is a tough one to diagnose for me. I don't use MM for base flesh painting but, I do use it on uniforms and equipment...
1.) Use a flat #2 sable brush. If the one you are using is a round pointed one, it just may not work as well. I use a flat #2 for base coating with oils.
2.) Try short - almost "stabbing" brush strokes to minimize paint marks.
3.) Paint to thinner ratio? 1-1 or 1-2? Paint to thinner. Believe it or not, several THIN coats are desirable. Did you try more than one coat yet?
4.) Primed - I am really surprised that you are having a problem with a primed figure.
Also, for next time... ALWAYS practice on a dummy piece or cheap-o Italeri figure BEFORE you commit to your nice expensive one. I cannot tell you the number of times that I have averted MAJOR disasters by this simple act.
Let us know how this works out for you! Good luck