Airdrop

I have an Imac (mid 2011) that has airdrop showing, however, on purchasing a new Imac I am not able to airdrop between them? The new one has picked up phone and ipad no problem but I can't get it to work on the old one. I have seen somewhere that I should be able to do this can anyone throw any light on this please? I have done the usual turn off wifi and blue tooth, restarted mac etc, any help 'greatly' appreciated