Ensure they are using the same firmware version.
Recommend to use 1.4(3)SR1 / 1.4.3SR1.2, which is the latest on Cisco.com
You can test the radio by going to Settings > Phone Settings > Diagnostics > WLAN.
Ensure you are in range of the desired AP and should appear in the site survey screen.
If you continue to have an issue with only 1 of the phones, then open a TAC case and they can perform some troubleshooting with you to see if the phone needs to be replaced or not.