Ipad 2 not connecting to airport express

Hi have this very strange issue.

I have a wifi network with an airport extreme as main wifi unit and an airport express as wifi extender. My macbook, iphone 3gs, ipad 1 work perfectly fine: when put in the "low airport extreeme signal area" they show full signal because get connection from the airport express. My ipad 2 does not. I tried tinkering with settings but no success. Any clue?