Why does my watch not calculate my standing time on my exercise ring?

Standing will close your blue ring, your green exercise ring can only be closed it you move for a longer time and your heart rate goes up a bit, plus your arms are moving.

Apple Watch - Close Your Rings - Apple

Track daily activity with Apple Watch - Apple Support

The Apple Watch does not really know what position your body is in.

The Apple Watch uses the accelerometer to determine that your Apple Watch arm is swinging as you are walking.

It takes between 30 and 60 complete cycles of back and forth, normal walking, arm swings in a short period of time before the Apple Watch decides you have accomplished your “Stand” for the current hour.

If you interrupt the arm swinging for a few minutes, it will reset, and you have to start the 30-60 swings over again. Later in the day, it seems to require more arm swings, or if you keep having incomplete stand attempts, in the Apple Watch’s opinion.

In reality, you can just gently wave your arm around 30-60 times in a short period of time and get stand credit.

Knitters, sitting in a chair, always get their stand credit and huge step counts.

Professional Chefs, on their feet, working in a kitchen all day, miss their hourly stands, because their Apple Watch arm is often holding food being chopped, or carrying something when walking, or holding a pot, pan, etc…

No 30-60 arm swings in a short period of time, then no stand credit.