No audio if more than one person unmuted

We use Zoom to run morse code lessons.

When we join the meeting we can all hear each other.

I take the Host, mute and start sharing a morse code program or webpage to send the code.

When the code is finished, I reveal the code that was sent, unmute and sometimes I cannot be heard until I stop the share.

More lessons are sent and then usually we have more audio problems. If more than one person is unmuted we cannot hear each other. We have to get each person to unmute to speak and all others muted or we cannot hear.

Does anyone have any hints of what we can do to overcome this?

We have to have background suppression set to low to enable the morse code to be heard and all our mics and speakers are working fine when we join the meeting, it is only after screens have been shared that the problems appear.