Hi @Trekko!
The "malfunctioning" join button is usually an issue of either:
A - If you are trying to join on a friend and the game you're trying to join is full, the join button can disappear/become unclickable. You are still in queue and will be put in game when there are sufficient spots on the team that your friend is in. This is just a visual bug.
or:
B - Very rarely the join button can appear and remain after one of your party members has left a game. From what I know there is no solution to it right now, other than to have the affected person restart the game. ☹️
Hope this helps a little bit atleast! 🙂