I have an iOS app that worked well on all devices in iOS 6.
I recently upgraded my xcode to version 5.0 and tried to build the same app for iOS 7 on an iPhone 5.
Assuming you are actually targeting only iOS 6.0+, it sounds like an issue related to upgrading Xcode. Sometimes, things can go awry when transitioning to a newer Xcode version.
Here are some general tips you can try: