Hi, forum noob so please be gentle

I am having trouble getting a domain transfer to go through. Someone has bought one of my domains which I have registered at 1&1.co.uk. He wants to transfer it to his account at joker.com, we have tried to transfer twice now, but both have failed.

I have cancelled the domain at 1&1, and have called them to check and they confirm that the domain is 'active and unlocked, ready for transfer'. When I get the two 'Request to transfer a domain..' emails (owner and administrator), I click on both links and click 'Yes' to authorize.

The last try was last Wednesday, but the transfer hasn't gone through. Joker.com have told the buyer:

"We are still waiting of an ACK from current registrar: SCHLUND+PARTNER AG."

1&1 tell me that they didn't recieve a transfer request from joker.com.

So, any help or advice gratefully recieved!