My iPhone 4S says no storage available even though there should be space available

The first time I got the message, I was willing to believe I just needed to free up some space, so I uploaded all my photos and cleared them off of my phone and deleted a few apps I no longer use, which should have opened 2-3GB in storage space. But the phone still shows 13.5GB used, 0 bytes available. When I add up the usage - 6.6GB for facebook, 1.1GB for music, and 1.45GB by other apps and games for a total of 9.15GB - it does not total the 13.5GB the phone shows in use. I don't suppose there is an easy way of fixing this?