Changing WLAN network password on Airport

I have just got a new router, in order to overcome interference and other problems, a nice little D-Link running 802.11N, and it has sorted out a whole slew of problems. One I can't overcome is that Airport on my wife's powerbook seems to be stuck with the original password for the network. (I'm using the same network name, for reasons). The password has been changed, obviously, and I will be changing it regularly, obviously. However, I cannot make the changes stick. Whenever the laptop is fired up, there is a failure to connect. I have to go into Airport, select 'other' and reset up the network and password, it connects fine. However, log out, and you have to go through the process again. I can, of course, change the network name and start again, but that would be deeply unsatisfying, as I am sure there must be a way to just change the password, and make it stick.