After the update on my Macs, an iMac Retina 4k 21,5” 2017 and a MacBook Air M1 2020, from Mac OS Big Sur to Monterey, I had this problem with AirDrop, not working between the two Macs or my iPhone and my iPad, both with iOS 15.1.
I have installed AdGuard in both my Macs and I found that the ‘guilty’ is it.
Reading this post in Reddit https://www.reddit.com/r/mac/comments/qqqvvp/macos_monterey_airdrop_not_working/, I could solve the problem and now all devices work well.
If you have installed AdGuard too, give a try :
open AdGuard settings -> Gear -> Advanced -> Advanced settings -> Change network.extension.monterey.force.split.tunnel to true value (it’s false as default).
I hope this is the solution for many of you.