Long distance sims can’t visit

I'm having trouble getting my sims' friends from University to visit. Every time a sim invites someone over, a notice tells me they'll arrive at 9 AM the next day, but they never show up. If I try to invite another visitor on the same day, I get a notice telling me that there can't be more than one visitor so I know the game his registering the command, even though no one comes. What can I do. They stay greyed out