Thanks for your post and welcome to the community. I am very sorry to hear that you are having this issue, however I would love to help.
Firstly, I would recommend ensuring that they are correctly fitted in your ear by using the correct Stayhear+ tips. The heart rate sensor is located in the left earbud.
I would also recommend deleting and reinstalling the Fitness app.
Please update us on the results of this.
Hector B - Community Support
Thanks for posting and updating me on this.
Please close and reboot both apps. I would also recommend unpairing and repairing the headphones with your device. This can be done via:
I hope that this helps and I look forward to your response.