There is a bug that stops guys from playing if you have a lower end team (possibly after a loosing season) with a high end player.
Mike Williams is "holding out" on you.
In previous Madden's players would hold out and demand a trade if they were unhappy with their team or their contract. That feature was removed, but the backround programming logic is still there.
Unfortunately this bug was never fixed after 2 versions of the game, and 7 patches.
Happened to my Detroit Lions team when I signed Mario Manningham... He played one season and then wouldnt show up on the field for his next season even though he was on the roster #1 spot.
Im sorry ot tell ya, but there's nothing that can be done. Bugs like this (and the IR bug) either can't or won't be fixed for some reason.
*shrugs*
Trade him. Or this.