Preferred network available, but still prompted

Very frequently, when I wake my Macbook up, I’ll be prompted to select a network, as “none of my preferred networks is available”. Yet, my home network, very much a preferred network (I can see it in the list of preferred WiFi networks in Network Preferences), is right there in the list, and my computer won’t connect unless I select it.

How can I get my computer to recognize my preferred network (which it is clearly aware of) and stop asking me to join it?

I’m running Mountain Lion v10.7.3 on a Macbook Air, and am happy to provide more information as needed.

Thank you.