Airport/Sleep Mode problem

I just recently started having problems with my wireless internet. Everytime I close my powerbook, and come back 30 mins or so later, It's not getting the airport extreme signal. So then I have to click on my network and sign back in. Has Apple put out a firmware update in the past couple of weeks or something that I need to install, or is there another way to check this? I do have it configured to sign on to my network automatically, so what could be the problem?