Hi
Check that Wrist Detection is turned on (on your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection.
Tracking can be affected if your watch loses contact with your skin, causing it to believe that it has been removed from your wrist. A loose fitting band can trigger this by allowing your watch to lift from your wrist and/or by allowing clothing (for example, the end of a sleeve) to slip beneath the back of the watch. Sensor performance can also be affected by temporary or permanent changes to your skin, such as some wrist tattoos.
Check also that the back of your watch is clean and dry (use a nonabrasive, lint-free cloth).
During exercise, Apple Watch should be worn snugly on the top of your wrist. Apple suggests that you consider tightening your band before workouts and loosening it again afterwards. Better contact with your wrist may also improve heart rate sensor performance during your workout.
Workouts may also be paused or ended early due to accidental interactions with your watch's screen (including by clothing). To help prevent this, you can lock the screen during workouts (see the Workout app article for instructions).
More information:
Use the Workout app on your Apple Watch - Apple Support
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
