Did you make sure your region matches to your games?
If you happened to download the DLC from US or Japan, but your games are European (for whatever reason, accident whatever) - The DLC will not work, or other way around, you have US games but you accidentally got EUR DLC. The DLC needs to match the game.
Third and quite final possibility is you did not install the DLC to right destination. Say the games are on USB drive but you accidentally installed to internal memory. The DLC needs to be in same place as the game, or it will not work. Under system settings, Data management the "Add on content" box needs to be within other data of the game. If your other storage has an extra entry for your games like BOTW, and you press them in the menu and only thing there is "Add on content" - Remove it. Do not try to move it as Wii U system itself never anticipated these happening, eShop application would automatically see where the game is and install it to the same place. Trying to move or overwrite the DLC etc. over your actual game and saves might remove that game data and saves, costing you reinstalling everything and even your saves. Just remove a lonely DLC entry and reinstall it to the right destination with WUP installer (installation does not wipe anything, just writes additional data to same folder structure)
If it is not any of these then there is something you missed, even if you are sure you didn't (Update, etc.)
also, if anyone wants me to make music for a rom hack, just tell me the game and I'll get the soundfont to make a soundfont cover of a song, I've already made a few in the Wario Master of Disguise soundfont