Bood Oxygen Tracking

I could be that it's not snug against your wrist.

Mine doesn't work if my wrist is twisted a bit-- it's sometimes doesn't work when I twist my wrist to look at it, but it works when my wrist is pointed away. This all seemed to change when I switched bands.

There's some good information here, about half-way down:

How to use the Blood Oxygen app on Apple Watch - Apple Support

Yeah-- my problem was the opposite-- it worked when I walked with my arms down, but it didn't always work when I did it manually, because I twisted my wrist to look at it. Crazy.