iPhone Storage Space Inaccuracy (No Storage Space)
Sometimes I have seen a lot of space taken up with the "other" category in the graph of space taken up on your iPhone in iTunes. Fixing that is not actually that difficult.


Back up your iPhone to your computer (I would encrypt the backup as the backup will then save all your passwords too, which it does not do if you don't encrypt the backup.)
Once done go into Settings > general > reset and erase all settings and data. (you may have to first turn off Find My iPhone, if its on, under iCloud)
When the phone reboots, plug it back into the computer, launch iTunes and restore the phone from the backup you just made.


Usually when you do this, if there is a lot of temporary or unneeded data on the phone this will do some house cleaning and free up some space. No guarantee this is your issue but it is easy enough to do and you won't lose anything except maybe an hour (or less) of your time.