Can't preview my map inside Fortnite

Hi !

Apologies in advance, sounds like a stupid issue, but I can’t seem to get a preview of the map I’m working on in UEFF inside Fortnite. I click properly on “Launch Session”, the game launches, but I just stay stuck in the lobby (and the editor seems to think that I’m in).

Any clue as to what’s going on here ?
Thanks !