The most common reason for this issue is that Cardiogram doesn’t have permission to read data from HealthKit. 

Here’s what you do to fix this: 

1. Open the Health app on your phone
2. Tap the “Sources” tab (Second from the right on the bottom of the screen) 
3. Tap “Cardiogram” in the apps section
4. Make sure all of the sources are on, i.e.: