Airdrop between two iMacs
I am trying to set up Airdrop between two iMacs. One is running Catalina, and the other (an older iMac) is running High Sierra. I have wireless turned on and the computers are on the same network. Bluetooth is turned on on both computers. At one time I could "see" each computer in the other's iMac Airdrop display but now, I run Airdrop on both computers and neither shows up on the other. (I can see my iPhone on the iMac running Catalina, but not the one running High Sierra.
What could be wrong?
Thank you.
iMac 27″, macOS 10.13