I had the same problem, but turning off the phone completely (not just rebooting it), and turning it back on after a minute fixed it. Previously, I went through Apple phone support with no success and a Genius Bar appointment, at which they concluded that the issue was corrupted software and that the only solution was a factory reset and a reinstallation of all my apps and data. We couldn't even do a reinstall from my iCloud backup, they said, because it would reinstall the software glitch again. I was at the Apple Store for my third appointment on this issue, with an external hard drive in tow so that we could drag my data off the iPhone before we reset the machine. On a hunch, the tech did some diagnostics that showed that my RAM clock speed was off. He asked me how often I shut my iPhone off. I told him "never," but that I reboot it from time to time, but he said it's not the same thing in terms of the iPhone's allocation of RAM. Turning it off and on a minute later did the trick — and saved me from having to spend much of my weekend resetting up all the apps and data on my iPhone. Incredibly simple solution for a problem even the first two Apple support guys couldn't solve.