Only 1 session with 12 users and video participants start to drop out


I checked host and participants side during a session with 12 users.
From more than 10 users, video of participants start to drop out.

I checked server resources (2 cpu, 2gb mem, 5gbps bandwidth) and none of them were exhausted.
Also on host side and some participant sides I checked up- and download capacity which was way enough.

Any setting in particular I overlook?