is it really a mod, or some other category of items (asset, map themes, etc)
post link of the item
only mod need to be turn on manually, everything else enabled by default

then check if the item really is downloaded
SteamLibrary\steamapps\workshop\content\255710
the folder number same as the link of the workshop page should exists

example : steamcommunity. com / sharedfiles/filedetails/?id=2858591409
will create a folder "2858591409"

I am having the same issue. I downloaded move it and it is showing in the folder with the correct number like you mentioned above. I then go into city skylines, content manager, mods and it is not showing for me to enable.

its in Assets
i figured it out finally
all mods appear in assets when downloaded succesfully

some may be in mods but buildings, cars and such are seen as assets maps are in maps and mods are besicly other mods that dont really have a subject