Does not automatically connect to recently used WIFI networks

Question

Recently purchased 13" Macbook Air, with Lion installed. All's well, except upon start-up or waking up from sleep mode, it does not automatically re-connect to the WIFI network it was just connected to. Instead, I'd have to manually find the network to select it. (Incidentally, some of my friends who also use Macbook Air and Macbook Pros have also experienced this periodically.)
Another strange thing, is that there is this one particular WIFI network at work, that I've successfully connected to before, upon return the next time, all of a sudden, the network isn't even visible anymore. My OS5 iPhone sees it fine, and never has a problem, but repeatedly this happens. Other people at work who also use Macbook Pro and Macbook Air also encounter the same problem. People who are using PCs don't have the same experience.
Is there a setting somewhere? I have "Remember networks this computer has joined" checked, and I see the particular network listed as ones I've joined.
Any light that can be shed on this would be greatly appreciated. It's very frustrating, especially Airs don't have a built-in ethernet port. I hate using the adapter, it steals one of my USB slots.
Thank you in advance!

Answer 1

Hi sashton,
When troubleshooting Wi-Fi connectivity issues, I find the following resource has the most relevant information:
Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6
http://support.apple.com/kb/HT4628
Furthermore, you may want to avoid giving your network a common name. While the following article (and excerpt) refer to iOS devices, the same applies for your Mac:
Avoid giving your wireless network a common name (SSID)
http://support.apple.com/kb/HT4450
If you were to give your wireless network a common name (SSID), users of your network who use iOS devices [sic] and are away from your network would run the risk of joining a different network with the same name. Having joined the unexpected network, they would be exposed to potential data theft.
Because of this security issue, Apple maintains a list of common or ambiguous SSIDs, including the default SSID for many commercially available access points, as well as the SSIDs "wireless" and "default". If your network's name is on the list, any iOS device [sic] that joins the network stores the access point's BSSID and associates it with the network's SSID. Subsequently, it will only auto-join the network if the access point's BSSID matches the information the device has stored. This avoids joining a different network with the same name.
Therefore, you may want to refer to the following article when configuring your Wi-Fi router:
iOS and OS X: Recommended settings for Wi-Fi routers and access points
http://support.apple.com/kb/HT4199
Thanks,
Matt M.