I was trying to add iCloud support to my existing app, but found that after adding entitlements and configuring iCloud, my app would no longer debug.
I realised that my generic iOS development certificate had a different APPID from the app I was working on. So to fix it, instead of using my generic certificate I created a specific development certificate for that APPID.
I refreshed my provisioning profile in XCode, cleaned out the app, disconnected my device, restarted XCOde and connected device and ran, and it now works a treat!