Sound quality on live voice teaching sessions

I've been teaching voice over Zoom. I use an iMac, sometimes with the internal microphone sometimes with my Yeti stereo microphone. My students can hear me up and down the scale when I demonstrate and sing above C5. I cannot hear them generally above C5 and the sound cuts out. They have done all the suggested remedies for music on Zoom and mic adjustments on their side. Does anyone have any suggestions?