I read that AirDrop uses Bluetooth to make machines handshake then transfers the actual data over a p2p wifi network. But I vaguely remember it also working over existing networks, even with ethernet, which may have been in an older version. Either way, it's complicated and thus unreliable. I'm guessing Bluetooth is the weakest link there, cause I have a hard time even pairing speakers with that crap.

Check if BT is enabled on both your phones.