When you go to Biff do you still have the "earn $5000 for Victoria" quest open in the top left corner? If not then you may have done the blowjob for Steve. Only one of the blowjob or a photoshoot can be done each week. Also sleep and see if that quest opens up on Saturday morning. Another possibility is that you have not yet acquired the red dress. There is a point in ep 2 where Biff will not hire Monica until she turns up to work in something better than her whore outfit. What exactly is Biff saying when he turns Monica down?