there are some things that can cause bad rating, no one knows for sure what but there are many things people suggested, such as:
- renewal of contract will improve rating.
- having high morale & condition.
- playing the player on his natural position.
- training the player's physical & mental.

not sure about the first and last one, but the others will improve the rating for sure. if the players still gets crap rating there's nothing you can do about it, you can wait until it gets better or just sell him.