Or they are caught in that loop hole where the game has already lined up which Sims will show up at your house to be your new roommate..but you have want to ask someone your Sim knows...
It's a mess. You have to play it just right at the right moment to get a friend to actually be able to live in your home as a roommate.
Before I enabled roommates (option is not available sometimes on a home phone) for the house, I would invite my friend over..then enable over the smartphone. Close smartphone, and enter it again to set the amount of Sims you want as roommates. (That should have all been in one easy step but it isn't.)
The immediately turn and ask the Sim at your house that is your Sim's friend to be a roommate. Before, as fast you can, the game lines up which Sims in the world will arrive later that night or the next day.
Once the game determines who will be the roommates, nonplayables, it's too late and very hard to get it timed just right.