It could be that you're not waiting long enough for the add-ons to download or it could be that for some reason they're not downloading from Steam or the game is failing to write the files to your hard drive due to admin access being required or something. Have you tried launching the game via the .exe and choosing Run as Administrator and tried turning off any firewall or antivirus software that could be blocking the game from completing the updating process.

If you yourself have ever uploaded an add-on it will download any of your own published add-ons if they exist on the cloud. You can use the Workshop manager and use the Tools > Cloud Information button to see what files are stored on your cloud.

I use Microsoft Essentials and default MS firewall, it doesn't seem like a problem caused by them.

I can't even upload screenshot to Steam. There are people reporting that they can't upload screenshots. I'm guessing something is up with Steam Cloud servers. But who knows.

edit: I followed this article and it didn't solve anything https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638
So now I'm gonna try reinstalling Steam without deleting SteamApps and Userdata folder.

reinstalling Steam did not solve this either. plus same thing happening for L4D2 worksop items as well. this really looks like a problem caused by Steam client or just something from Steam's end.

Same problem here. It worked for me before, but now it doesn't.

I have the same problem!

same for me also

Well I don't know what's going on guys but appreciate the feedback. Appearantly a lot of dudes having this problem. I mean I can't even upload a freakin screenshot lol it's not like I spam the ♥♥♥♥ the out of but having no go is really pissing me off anyway
peace

I had the issue myself last night, not sure what is causing it, seems to me like it might be on Steam's end of things not returning messages to the game or allowing downloads. I'm not sure what I did but I came on this morning and the issue has resolved itself. The game was updated on the 18th and no code was changed for that update and the problem didn't show itself until the 21st so I don't believe it can be on our end. :S

Alright it seems things are back to normal chaps probably Steam is sorted this.

Problem solved. Thanks for the reply Dec1234 :)

Yeah it came randomly, days after the update. I read about it from you guys and was freaking out a little (because nothing changes in-between), then I noticed it happened to me one night then was gone the next. Phew!