Sometimes when I go to join some challenges, I click "Join" but nothing happens. Usually it happens on Duos. Reloading the game and re-logging in doesn't fix it. I just can't join that particular challenge and have to wait for another one.
https://www.youtube.com/watch?v=C4i5fGcEs9s
https://www.youtube.com/watch?v=C4i5fGcEs9s