hmmm...all you did was enable WPA and no other changes? Then 2 things.
1 is to make sure the wireless card of the computer not working supports WPA. Some older and cheap odd brand name ones don't, as they may not allow the firmware to be loaded to support WPA(even though there is no physical change to the hardware necessary). If it's actually WPA2, older hardware will not support it, even if the chip allows loading of newer firmware, as this requires an actual change to the chipset to support AES.
2 is to make sure the keys match. Passphrase mode sometimes will not work right at syncing keys. You may need to enter the actual key on the host and the client, if this happens.
And oh yes, make sure the driver version is new enough to contain firmware that supports WPA, if your card is an older card.--
The new Sony rootkit-Using the ability to remove features you paid for. What's next? Boycott Sony products »[Rant] ps3 update = no more Linux