DarkStar helped me by providing this information:
Have a look at
https://www.ikmultimedia.com/faq/index.php?category=11&topic=sampletank-4 then "I can't find or load my Virtual Instrument's sound content library, what can I do?"
There's a folder hierarchy pic in there.
Make sure you've unzipped all the files for the entire instrument group. Some of the ZIP files just contain files that go in the "SAMPLES" folder while others have "INSTRUMENTS", "LIBRARY INFO", etc. Sounds like you may be missing some of the Sample files.