You need to use the “migrate” function to get all dependent textures/materials/etc.
What I do, is create a separate project that just has all the packs I want to take things from, added to it. Then I open up this project, and “migrate assets” (right-clicking in the content drawer) into the “real” project that only wants some assets.
You have to either run two editor processes (make sure you have enough RAM!) or re-open projects to go back and forth.