If you can.. try to make that a prefered connection..
Have you update the firmware (Extreme/express)?
Both extreme/express have so many configuration option... you should look at it carefully.. and read the help file..
Have wireless network of airport extreme connected to internet and airport express as extender. Both set up as base stations using same name and password. This worked fine with iBook - I can move throughout the house and the iBook will connect to the stronger signal without any problem. Have new hp dv1000 with xp home SP2 and it will only connect to the airport extreme, even though the computer is normally used in a room that is much closer to the express. Have run Network Stumbler and the express has a very strong signal. Why won't the HP connect to the stronger signal? It keeps dropping the connection to the weaker signal.