The event you are attending has fewer participants. The system tries its best to pair up new folks during every breakout but depending on the number of participants and the number of breakout games played it's possible you've rotated through everyone already. Enjoy that extra time with your match!

The breakouts have A/B sub-matching and the group sizes are uneven. When A/B sub-matching is enabled the system prioritizes matching up participants from subgroup A with participants in subgroup B. If the group sizes are uneven, you're more likely to be matched with the same person of a different subgroup.

You or your frequent-partner (or both) are joining the activities after the initial countdown. All participants who join during the countdown are placed in matches, and if they join soon after the countdown ends, they may get placed in a preexisting match with others who joined on time. However, if they join much later, they can only be paired with others who joined late. If you and your partner are the only people who joined substantially late, and everybody else was already placed in matches, there may be nobody else for you to match with!

You or your partner’s internet connection is weak. The resulting latency pushes you past the “Join Activity” countdown, which is more or less the same as joining the activity late.