@cndspec - my daughter also says that notifications are spotty (although more frequent than George Ponder just stated.) She keeps her phone with her all the time (typical teenager) but suspects the alerts stop only when Bluetooth disconnects (similar to how the Band operates, although I've found the BT connection stays on pretty consistently so long as I have my phone with me all the time.)
Have you figured out how to keep the BT connection continually active between the HR and the Windows Phone?
I should note that she has multiple BT devices also - music headphones, running ear buds, car speakers, etc.