can't use WPA/WPA2

I've a iMac 27", i5 2.7G, 4G, running Lion 10.7.4.

When I set my router to WPA/WPA2, it sees the router's name and understand that my router needs WPA/WPA2 password, but it doesn't connect to internet. If set to WEP, everything works fine. Any idea what's going on?

I've connect an iPad, iPhone and two other PCs, they all work fine.