Only one app at the same time can use a heart rate sensor over Bluetooth. If you already started an app which tries to connect to your heart rate sensor (like e.g. RunKeeper, the Poler H7 app, ...) it will make a Bluetooth connection to the heart rate sensor and the Heartbeats app will not be able to make a Bluetooth connection. In order to resolve this, you should force-quit all apps which connect to the Bluetooth heart rate sensor, or restart the iPhone and start Heartbeats prior to any other app which connects to the Bluetooth heart rate sensor.