Verify mod file and archive are present in folder ~\Documents\Paradox Interactive\Crusader Kings II\mod. Note that Steam cloud sync appears to be broken for mods with capitals in user_dir.
Make sure you have Steam Cloud enabled. In case no download progress appears in Mod tab, check the setting for downloading while game is running ( Steam -> Settings -> Downloads -> Allow downloads during gameplay). Try subscribing to the mod in Steam client, while CK2 launcher is already started. Verify you haven't reached the 50 subscribed mods limit. If the mod doesn't show up in the launcher: Once the download has completed, tick the checkbox to activate the mod. At the same time a "Crusader Kings II - Workshop Content" download will be triggered in Steam client (where you can monitor progress). Under "Mods" tab, the new mod will appear at the bottom of the mod list as unselectable, with a red background (which does not reflect download progress !). Find the mod on the CKII Steam Workshop. mod file already exists with same name under ~\Documents\Paradox Interactive\Crusader Kings II\mod\ it will conflict during the installation.
Important: Remove any previous manual installation of the mod if any.