If this is a widespread issue, Steam will eventually refuse to download any new mods.
!Workshop folder contains corrupted files – It’s also possible that this issue occurs because some of the mod files that you downloaded have become corrupted.
If this scenario is applicable, you can verify if the mods are actually downloading by switching to Big Picture Mode.
Mods status bar only appears in Big Picture Mode – For some reason, Steam changed the default behavior, so in the latest versions mod updates will download silently in the background, so you can no longer track their progress.