Apple Watch doesn't recognize standing

I like to standstill and juggle 3 beadbags. It takes me approx 60 changes in arm direction to get stand credit (your mileage may vary). It may be possible your stand goals differ

The Stand ring shows hours in which you've stood and moved for at least a minute. Complete your daily Stand goal by standing up and moving around for at least 1 minute during 12 different hours in the day. Even if you stand all day, you still need to move around.

If you specify that you use a wheelchair, the Stand ring switches to the Roll ring. Roll shows hours in which you’ve pushed around for at least 1 minute.

See this URL for changing your Stand Goals

https://support.apple.com/en-us/HT204517

Move your arm around this helps

It only senses the acceleration and deceleration of the Apple Watch on your arm.

You stand on your head, lie on your back, sit in a chair while waving your Apple Watch arm around, and you will get stand credit, as well as step credit.

Knitters get fantastic credit for Steps because the needle movement is all it takes to fool the Apple Watch that you are standing and walking around.

I wave my watch arm from my elbow up and down towards my shoulder for 1 minute whilst sitting down,so far it's worked every time.