My colleagues and I had trouble AirDropping files from our Macs to our phones. Sometimes it worked, sometimes it didn’t.
I figured out that AirDrop would fail with the Cisco AnyConnect VPN client connected.
Specifically, when your Mac’s sharingd service tries to connect to https://yourphone.local:8770/Ask to ask for permission to accept a file, it times out via the VPN. This happens even if LocalLanAccess is allowed in your VPN settings.
The solution was simply to disconnect the VPN client whenever you need to AirDrop something.