Breakout Rooms stopped working

I was conducting an exhibition for three days with many participants(>150pax) coming in and out the rooms. Initially, everyone was able to join the rooms, but not sure why those who came in later could not join the breakout room at all, all stuck in the main session. Interestingly, those who were already in one of the rooms, were able to switch between rooms. This has been happening for both days and I assume it is going to be the same for tomorrow. Anyone has this problem?