I was having the exact same problem (stopped showing app updates after 12/12 - I'm in the USA).
Anyway this is how I fixed it...
I had a number of really old apps purchased under an OLD (ok ancient) Apple ID. I went through and trashed every single app that was purchased with the OLD ID. Once all 'those' apps were removed (leaving only my current iTunes Apple ID pruchased apps) the" Check for App Updates" option worked!!!
How do you find apps that are purchased under a previously used Apple ID?
iTunes > Apps > List (view) > sorted by Purchase Date.
Find the oldest one, then right click and "Get Info". Look for Apple ID in the window - if it's your current Apple ID hit the "Previous" button.
Continue hitting "Previous" (letting the Get Info window refresh) until you find one of your Apps purchased with your old Apple ID.
Once you find one, close the Get Info window, then delete that App from iTunes. (I sent the apps to the trash because I have backups)
Continue through the ENTIRE list. It's important to go through the entire list because recently updated apps under the old Apple ID will be up higher on the list. Yes it's a pain, but it was worth getting iTunes back in order (showing available App updates).
Again, I was having this problem, but removing apps under my old Apple ID resolved this issue...