Tournament starts, some teams will disappear

I write what happens to me in the tournament.

I create as an admin a tournament for 20 teams. The tournament will fill. The tournament will start, but when the tournament starts 2 teams that have been entered will disappear. I do not understand how it is possible. Many people tell me they are enrolled in the tournament, and when a match table appears, the teams are not in the tournament.

You do not know how to fix it?

Thank you