Unreliable Heart Rate Monitoring?

I’ve had a series 2 watch for about 9 months now. Previously I had a Fitbit blaze and I had gotten used to the constant hr monitoring and I like to keep an eye on my heart rate while working out. My old Fitbit was always able to find and keep my heart rate and it was generally fairly accurate. I always hear about how fantastic the Apple Watch heart rate monitoring is and when my watch does show my heart rate it is indeed quite accurate.

My issue is that when working out the heart rate is a greyed our number about half the time. I believe this means that the watch can’t find my heart rate. I’ve tried moving the watch around, tightening the strap, everything. This happens no matter what workout I’m doing. Am I missing something? Is there a way to keep the watch from losing my heart rate?