Screen share on only tab not working (shares entire screen)

Hi all, we do a lot of webinars that we share a presentation and begin with music. We've always been able to share the tab for the presentation only (that way when we go to turn the music off the entire group doesn't see that process. They continue to just see our holder screen). However, it has stopped working. Now, when I check that tab only it seems to share the entire screen. I cant seem to get it to only share that one tab/presentation. Please help! Thank you!