Since the latest patch, on Xbox One if you press X to bring up the list of available LFG groups that you can join, literally nothing happens. There are others reporting that it's the same on PS4 when you press square.

On Xbox One at least, if you press the left trigger it brings up the list of LFG groups. I can't confirm whether this also works on PS4, but I suspect it might.

EDIT - Apparently pressing the left trigger also works on PS4.