Finder doesn't show files tagged on network drive
Having the same problem. Finder is not showing all the tags I made on various documents after I transferred everything to my new AirPort. 

So, a kind of a workaround (although far from ideal) is to create your own tags: create a Smart Folder in Finder to search (also) in your external drive (via 'shared') and let one of the search criteria be 'filename' 'contains' ... .  

Then you fill in a self-made 'tag', like 'to do' or 'review' or 'finished', etc. Then name this this Smart Folder for instance 'To do' and place it in your Favorites-section on the left side of the Finder-menu.

Finally, change the file names of the files you want to find and enter anywhere in the file name itself the same words, like: 'to do' or 'review' or 'finished', etc. 

Now, if you click on the Smart Search folder, you will see all files with for instance 'to do' in the file name, in your external drive.