Hi. I've never encountered this problem until recently. I often watch videos on my phone and my storage (16GB) is usually close to empty. I sometimes download videos directly to my phone through wifi and previously nothing has gone wrong.
Recently, whenever I've been downloading and I've gotten below around 800MB of space, files have been disappearing of my device. No notification or anything has come up to tell me and I have no idea why this is occurring.

Does anyone know why this is occurring or any way to fix it? If not, would returning my phone to factory settings work in fixing this problem? It's not ideal, but I'd be willing to do it if nothing else works.
Thanks for your help!