Why won’t my Apple Watch track my heart rate during an outdoor walk? It works during my tennis workout.

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

That didn't work for me. It will randomly just stop detecting my heart rate during workouts. Any tips on how to prevent that?

Tighten the watch band a touch and don't wear the watch on top of any tattoos.