Laptop does not detect wireless from Zyxel 660 router

I have a Zyxel 660 router from Post Office broadband. My Iphone and office laptop (dell winXP) get the signal from the router and i can access the internet, but the other two laptops (lenovo and HP, winXP) in my home and a Nintendo DSi do not manage to catch the signal, it does not show up at all. The internet works fine when connected directly to the lan cable. I have checked all the configurations on the router but i cant seem to find the problem. anyone else has had any issues like this?