There is a know limitation on iOS that limits phone to watch communication to the foreground application (more details here). If your device is running httpebble then you will have to stop httpebble in order for Pebble Snap to connect to your Pebble Watch. Pebble Snap will automatically disconnect when switching away to another app and reconnect when opened again. To manually disconnect press the camera icon in the iOS app and it will turn red when disconnected. If the Pebble Snap watch app is open on the Pebble watch it will display and vibrate on connect/disconnect.
iOS 7 is not fully supported. A new version of Pebble Snap will be released after iOS 7 is released to the public by Apple.
As reported by a Pebble Snap user on iOS 7:
Voila, taking pictures now works smooth as butter on ios 7. The issue seems to be that once you launch the app, and then install the watch face, the app does not attempt to reset a connection to the pebble.