Zoom: My mic cuts out when other people talk, only when I zoom from my iMac

When on Zoom with 2 or more other people (so there's three of us in the meeting), the other people cannot hear me if they are talking. It is as if my audio feed is subordinate to everyone else's. My audio cuts in and out, which is not happening to the other participants. Multiple people can talk at the same time and be heard, but not me.

The only time I am heard is when I am one of two people talking. But if a third person starts talking, then my audio drops out of the call entirely.

Also, when this is happening, my Zoom frame/box almost never lights up.

In summary, if two people are talking, and I start to talk, no one can hear me and my box won't light up, until both of the other people stop talking.

If I am talking with another person, and then a third person starts to talk, my audio cuts out and my box stops lighting up. I can hear the other two, but they can't hear me.

I am using a 2017 iMac 21inch. It does not matter if I use the internal mic, or connect my AirPods. The result is the same. This does not occur when I am on Zoom with my iPhone or iPad.