Updates Won't Download via App Store

For quite some time, one of my computers has been unable to download the Available Updates it shows.

I can click on Update and for just a moment it displays a progress bar and "Calculating...", then it switches back to displaying the Update button. It is not downloading in the background because I can then let it sit for hours and nothing changes.

I was able to install El Capitan by downloading it seperately and installing it. Last week, I resorted to downloading the Combo Update, then I reinstalled El Capitan. Still the updates won't load.

I've tried Safe Mode, Root User log in, etc. I've repaired Permsisions, run utilities, toggled App Store Prefs... I still get the very same result.

Currently I can't get it to install the El Capitan Recovery Update 1.0 and can't find a separate download.

It's not a problem in my account because I can install updates on my other Macs without a hitch.

How do I fix this??