wifi problems

Hi

My late 2009 13 inch Macbook has developed a wifi connection problem. It won't connect to a secure wireless network and gives various errors or appears not to accept the password. I have twice taken it in to the local Apple store where, each time they have reformatted the hard drive and reinstalled the software. On both occasions, frustratingly, the wireless connection worked in store with the Apple unsecured wireless network but didn't subesequently function with any network I have tried which have WAP password protection. It is making me wonder whether there is a hardware problem. Any one had similar problems and/or can advise me?

Mny thanks

David