Conference showing concluded while still in progress

[Smore]

While a BBB conference is in progress and students are in it, another student will try to join but the conference is listed as concluded.  Teacher and other students are still actively participating in the conference, but other students cannot join.  Any idea why this is happening or how to fix it?  This has happened twice in the last month. Note- sometimes the student attempting to join has been in the class, but was kicked out for some reason, usually related to Internet connectivity.

Thanks.