Why does iTunes not update my apps?

Setup: iMac, El Capitan (10.11.1), iTunes v12.3.1.23

I used to be able to open iTunes, go to my Apps, choose updates, see all the apps with available updates, and click the update all apps button. Now when I do that, they don't update, and I get a series of error messages that the server timed out (one error for every app). If I try to update an individual app, the same thing happens. If I look at the list of active downloads, they are all stuck at zero kb of X mb. My internet connection is working fine, as I can open a browser and surf to my heart's content, email works, etc. Basically it is just this one feature, which has forced me to do updates on the phones/iPad directly rather than on the mac and then sync. Any ideas? Thanks.