Galaxy Tab A (2016) always shows full memory

My tab always warns of dangerously low memory. It shows 15.4 GB used of 16gb even though the sum of the images, video, documents and apps is only around 2.5 GB (per attached image). I've tried disk clean up, wiping the cache partition, deleting files and apps, but the amount of used storage doesn't go down. Has anyone encountered this before or can anyone suggest a possible fix?