Apple Watch ECG app not showing heartbeat line

I recently upgraded to the new Apple Watch. I use the ECG app a lot as I have a heart condition but on this new watch, when using the ECG app, it takes the ECG but doesn't display the red line showing the heart beat while it's doing the ECG, just the count down. Afterwards the ECG is viewable in the app on my phone and I can see the heartbeat line then but I can't see it live. I have had a look through the settings and can't see anything where this is optional. Can anyone help?