Unusable cleared memory
I have a 15 month old 128 GB iPhone 6 on iOS 9.2.1. I completely filled the memory and then deleted several movies and videos. It then showed 23 GB of empty memory. However, when I now sync to iTunes, sometimes it shows the 23 GB available and sometimes it shows less than 1 GB available. It changes back and forth when I sync. When I try to use the memory I freed up, it won't let me. How do I persuade the phone that the memory is freed up and usable?
iPhone 6, iOS 9.2.1