Unable to share my screen with more than one person

Hello. On Friday I arranged a jitsi meeting during which I had planned to share a powerpoint with the twelve participants. I rehearsed with a friend beforehand and that worked well but when I tried to share with more than one person during the meeting nobody could see the presentation, just a red dot. Could anyone advise as to how I can share presentations with more than one person. Thanks. Regards. Diane