Well yeah, this is a little cryptic. If you already found the cache and you try to update or download the cache, you get the error message 'not found or premium only'. The message comes from Groundspeak, because the call includes a filter which says: 'don't download those I have found already'. Solving this means no longer filtering out the caches found by you.
I'm happy with this, but you might not be. If so, let me know in this thread.