Macbook pro no longer automatically connects to network upon waking up

Question

Since yesterday - for no apparent reason, i have to manually choose my network when i open my macbook. This is not a new computer or a new wireless setup. All has been working fine (with occasional hiccups with new hardware here and there, but mostly fine) for YEARS. And of course, as of 2 days ago, no problem. Everything is set to connect automatically and my home network is top of my list of 'preferred' networks. It does seem to be taking a few seconds longer to show up than previously, so maybe that's why it doesn't join without my help?
And if that's the case, what would be making it take longer to appear? Any ideas? Have reset everything. Plugging, unplugging, shutting down, restarting, etc. Did the reset button on the airport extreme itself, too.
I have the extreme, plus 2 expresses to extend my network. (also not a new set-up)
TIA

Answer 1

Try the following, in order, until (hopefully) resolved:
1a. Delete Preferred Network(s)
System Preferences > Network > AirPort > Advanced > AirPort tab
Under "Preferred Networks," delete the network(s) you regularly use from the list.
1b. Delete AirPort Keychain Entries
Launch the "Keychain Access" application located in Applications/Utilties.
Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
1c. Add Preferred Network(s)
System Preferences > Network > AirPort > Advanced > AirPort tab
Add the preferred network(s) using the "+" button.
Restart or log out then back in.
2. Move System Configuration Files
(Note: You will have to reestablish your network connections settings.)
Go to /Library/Preferences
Move the SystemConfiguration folder to the desktop.
Restart your Mac. (Note: OS X will rebuild the files that are now sitting on your desktop. If this doesn't resolve the issue, you can move the folder back to it's original location.)