It takes a long time but eventually I get the top center dark brown. The edges are always green
I have the shovel head pushed down as far as it goes, it even pushes the screen up a little.
Not that this'll solve your problem, but in my experience these things are related. Even tho not recommended, lately I've been leaving a small gap between the glass joint and shovelhead bowl (especially with the 14mm joint) and I'm getting more even abv. Post right up against the screen = green edges.