Is that the exact message in the progress box at the bottom of the screen? Or does it say "Waiting for confirmed funds"?
Discussion
Exact message
I asked a dev, his guess is that you have the "Coinjoin time preference" setting preventing you from joining while fees are higher than normal. If you have it set to "Days", "Weeks", or "Months", change it to "Hours" instead.
Thanks. Looks like I do have the coinjoin setting "minimize cost" which is probably the reason.