I found that if I was trying to airdrop between two devices signed in to the same ICloud account, then Airdrop would not work. I had Macbook Pro 2013 10.13.3, IPhone6 and IPad Pro. I could airdrop from IPad to Mac and IPad to IPhone but not the other way around. I searched and found I wasnt signed into iCloud on my IPad but when I did sign in I could no longer Airdrop - hmmm - I signed out of ICloud on my IPad and my Mac and stayed signed in on my phone. Happy days! Now I can air drop between all devices.