Several seasons ago, I tried to find out how fan support (FS) was calculated. I concluded that only the BB devs know and they are not telling (if someone knows of a post that gives the formula for how it's figured please let us know).
In terms of roster, Home grown + Longevity, figure into the equation and may be a problem for you. Of the top 8 players on your roster, none are home grown (you didn't draft them) and half have spent less then a full season with your team. I think you got a FS hit for each of those factors.