aptitude not seeing updates in repository

["Todd A. Jacobs" <nospam@codegnome.org>]

One of my sources is:

    deb http://ftp.debian.org/debian/ unstable main contrib non-free

but despite repeated "sudo aptitude update" commands, I was still seeing
old files (as in more than three weeks old) when running
"apt-show-version -a <foo>" on a package.

I added a kernel.org mirror, and things are back to normal, but I'd like
to know how I should handle this in the future. Is there a way to force
a full update, or to figure out why I'm not getting the most current
list of repository files?

-- 
"Oh, look: rocks!"
	-- Doctor Who, "Destiny of the Daleks"