Hello, Westie0506.
Thank you for visiting Apple Support Communities.
I understand that you are experiencing issues with not being able to connect your MacBook Air to Wi-Fi. Wireless troubleshooting can be difficult at times as there are multiple factors that can cause one device to experience this behavior but not others. I would need a little more information with the type of router you are using to provide a better answer. However, try adjusting your router per the recommend settings outlined below.
iOS and OS X: Recommended settings for Wi-Fi routers and access points
If you are still experiencing this issue, take a look at the troubleshooting article below.
Wi-Fi: How to troubleshoot Wi-Fi connectivity
-Jason H.