When updating DLC manually (via the SGP settings page, advanced section): please wait a few seconds (5 at most) to be sure that the DLC list has fully downloaded before navigating away from the page. I found out the hard way that there's no way to ensure that the DLC list fully updates if users navigate away from the page. The script should automatically check for DLC updates every 6 hours on page loads.
If you find that this still doesn't work, try a force refresh (Ctrl + F5, or Cmd + shift + R if you're on a Mac).
Should this repeatedly fail, I recommend nuking all of SGP's saved data (on the Advanced tab in the settings page) and doing a force refresh.
THIS ADD ON IS DEPRECATED WITH THE ANNOUNCEMENT OF SG V2. See http://www.steamgifts.com/forum/DCyWE/steamgifts-yesterday-today-and-tomorrow
I will continue to maintain the DLC list and fix bugs/tweak existing features, but I won't implement any new features.