I don't know if I'm the only one but I've owned the Series 2, Series 4 and Series 7 and all of them, all of them have had the same issue with heart rate monitoring during workouts.
The issue is that in the first couple of minutes after having started a workout, every time I check my watch, the heart rate reading is black. So you'd think it's unable to detect your heart rate. However, when I pause the workout and go to the heart rate monitoring app, it will measure it correctly and give me the reading/result. Then I go back to the workout app and it will show the heart rate reading. And I've noticed I have to do this 8 times out of 10. If I don't, I'll see in the end result that the first 20-30 mins of my workout no heart rate has been measured. And because of the fact it can read it when I go check the heart rate monitoring app, it feels to me this is one bad software bug. It's like I have to wake up my watch's heart rate monitoring every single time and it's so annoying. I hope this gets fixed.