Unstable connection for 69 participants -reached limits?

We saw that Jitsi meet can support for 75 participants.

We use the plateform for preaching: only the preacher switches on the microphone and camera.

Yesterday, we were 69 participants in the plateform, the connection was smooth for most of the participants (only 6 had troubles, but could settle after leaving and re-entering the room again).

Today, we were 68 participants in the plateform, in the beginning everything was well, after 30mn: connections crushed, remains 30 participants, even the preacher was cut. After 8-10 minutes, people could re-connect, after 10mn, 30 participants could be back for around 30mn, then the connection became bad.

Why such difference between yesterday and today?