You will find some players will work for your team and set up, while others, regardless of looking better quality wise, simply wont.

I've found from experience over the years that keeping a steady team with the same core amount of players for a few seasons seems to get better results. Its almost like there is a bit of long term chemistry coming into effect.

I've had 3 and 4 star players consistently outperform 5 and 6 star ones.