I had the same issues as here described, HRM worked fine on all devices and all apps, except on the iPad. On the iPad it connected to the HRM, but got no HR data.
This happened after I had to change the batteries on the HRM as old ones were empty. After that, the TR app on the iPad just gave - - as heart rate, while e.g. the garmin computer was showing HR.
What solved it was to fully reset the iPad. Restoring from a back up would just bring the problem back.