Samsung tablet Android 5.1.1. .thumbnails folder shows as .98gb so I run ccleaner (or Norton clean). Both show as DELETED yet, not much increase in memory. Reboot new .98gb file. I read that creating a .thumbnails FOLDER would prevent that - nope.
So why not a .98gb gain in memory when deleted?
ANALYZE disk
It will show
HIDDEN CACHE as one of the items.
Click " FINISH cleaning"
It will clean everything you checked.
To BE SURE it happened, just close app, reopen and you'll see that it's all gone.