Breakout rooms not fully working

I held a meeting recently when I divided participants into approximately 9 rooms of 9 people. They were automatically sent to the rooms (ie participants did not need to click on join room) but two of the rooms just didn't work. People were not put in the rooms. I could send some of them to rooms that were working and some of them reappeard in the main room but others were just left hanging. Why did this happen and how can i prevent it in future?