Graph heart rate not available in activity app

Hello all,

Today I went out for a work out with my brand new Apple watch (series 3; watchos 4.2.3) and iPhone 6 (iOS 11.2.6). I started an outdoor run with a 30:00 goal via the workout app and after ending it I checked the activity app on the iPhone to check my stats. Everything recorded well but unfortunately my heart rate was not recorded in a graph.

When I click the info button it gives me the message that the graph is not available. There heart rate has not been measured frequently enough. Make sure that the apple watch kan measure your heart rate and that the app has access to your heart rate via the settings on the watch or phone.

Now I find this odd. I assume that Apple's own work out and activity app has access to my heart rate data (something which I cannot change in the settings of the health app by the way).

The only thing I can imagine is that the watch is not able to record my heart rate continuously while it should. The sensors on the back of the watch are firmly placed against my wrist skin so a loose watch cannot be it. Shouldn't my heart rate be recorded continuously when my workout app is running and if not why not and what can be done about it?

Thanks for the help in advance and cheers,

Daniel W.