How do get rid of ghost data

Hello! So a while back I asked a Genius Bar member why my phone says there is no storage available but when you add up all storage the apps take up, there's still a good 5 gigs that should still be available. He told me it was something called ghost data and the only way to fix it was to back up my phone then do a hard reset on my phone and to set it up from the back up. That worked the first time I tried and it showed me 6.4 gigs now available, but shortly after ( I mean like only a couple hours) it went back to saying I had no data left. So I tried the hard reset again the next day and the exact same thing happened. After the third time I decided to just deal with it since each hard reset took soooo long. That was around 5-6 months ago and I just tried that process again yesterday with still the same results. If the "lack of" storage didn't interfere with Snapchat or my messages, I honestly wouldn't care, but it is getting very frustrating now. Is there any way I could fix this issue? And for example, I'm trying to attach a photo of my storage and it won't even let me do that. Thanks!!