GK and MC are the hardest to get good ratings from in my team(s)
Players do improve if they fit into their position and team mentality, some don't though. It's worth looking at the guys skill points that are performing well on your team and comparing against those that are not.
My GK generally are reading the newspaper and thinking about lunch, so they get 5's and 6's I have learned to live with, and the MCs as well get 6's and I try to improve them, sometimes it works... sometimes I sell them.