Here is a collection of Question and Answers derived from your comments

Q (iOS): Cannot connect Bluetooth transmitter (e.g. Polar H7) to the App

In some circumstances HeartRate+ PRO does not recognize a Bluetooth device which is already connected to other App. To solve this go to the phone Settings > Bluetooth, identify your belt in the list, click "info" and then dissociate. At this point your belt will be recognized by HeartRate+ PRO.