TL;DR If your computer is connected to Ethernet as well as Wifi, take the Ethernet cable out !
My Macbook just would not transfer files to my iPad or iPhone. Sometimes it would 'see' the device, but transfer would time-out and fail.
I had tried all the suggested solutions:
- Flight mode is Off
- Both devices on the same Wifi network with Bluetooth enabled
- OS up-to-date
- Discovery mode is set to Everyone or 'Contacts only' if the devices have each other as contacts
- Turned Wifi and Bluetooth off and on
- Open Airdrop folder on the Macbook
- Reset Nework settings
- Hard reset.
In other words: 'The Works', matter what I tried, nothing worked.
At my desk I tend to also connect the Ethernet cable even if Wifi is active and guess what:
Unplugging the Ethernet suddenly made Airdrop work!
Hope this helps someone