Health.app Medications – How to Access Medications in Health.app

Go to Setings > General > Restrictions and enable it.

Do nothing and re-enable it, the app should be back. On iOS 10 and later, you might also need to go to the App Store and reinstall system apps so be sure to try that as well.

Last - look for any profiles in settings in case someone managing your device has prevented use of that app systematically. You can generally remove the profile or ask them to reassign your device if you should have access to the health app and store health data on your device.

If the watches are different, and the phones are different, then the only thing that appears to remain the same is you.

You don't mention, but have you ruled out the possibility of there being anything about you? For instance, I remember reading that tattoos could prevent the heart monitor obtaining a clear reading, and there was some talk about people with very black skin also having difficulties. Example article here http://www.bbc.co.uk/news/technology-32537030

The other possibility is whether there is something with the tightness with which you wear it?