A friend and I have been playing a multiplayer game together for a while, but after some time decided we wanted to save and continue later. Upon later loading the game, I did find the save file once she loaded up the game, sent her an invite to join, but after accepting the invite, the game didn't put her in the lobby, and if I were to then start the game it'd just run like a regular single player session.

We both have the game on Steam, and both our versions are up to date. Does anyone have any idea how to fix this?