Group meetings no sound

I am having ongoing trouble with sound when I have more than one client. I have sound from one or two participants,but not the other. It has happened with two others, with three others, and with four. It is either I can’t hear or someone can’t hear another group participant. We have to use the phone to continue the meeting and everyone hears the meeting. It has been happening for over a month. Is there anything to address the issue? Thanks, Diana