I'll bet it's the wrong address.
Did your client actually show the transaction as having been received at one time?
Because of several double-spend methods, it is possible that you could have seen the payment (even appearing on blockchain.info) but then it never confirms and eventually disappears as if it had never been sent.
That risk is one reason why using the #bitcoin-otc Web of Trust (WoT) is a good habit to get into.
- http://wiki.bitcoin-otc.com/wiki/Using_bitcoin-otc#Risk_of_fraud
- http://webchat.freenode.net/?channels=#bitcoin-otc-foyer
- http://www.bitcoin-otc.com
- http://www.bitcoin-otc.com/viewratings.php