Airdrop starts but never progresses
Airdrop relies on Bluetooth and WiFi. Basically, you can try following listed steps to resolve your issue.

Toggle Bluetooth &amp; WIFI off, then on OR, enable AirPlane mode and turn OFF.
Check if you have VPN enabled or not?
Check if Airdrop is ON and allows connection for your contacts or Everyone on both devices.
Reboot both iPhone and Mac.