There are 4 different possible settings for use cache, only 2 of those settings will invoke mover, and only one of them will result in moving files TO cache. Also, mover can't move open files so just setting something to cache-prefer (the one that will move TO cache) may not be enough to get it done. Disable docker service, or at least stop the particular container, then run mover after setting it to cache-prefer.
And only 2 of the 4 settings for use cache will result in files staying on cache.
See this FAQ for more details about the use cache settings:
https://forums.unraid.net/topic/46802-faq-for-unraid-v6/page/2/#comment-537383