Stand may be a slight misnomer. You have to move your wrist wearing the watch as well. In fact, I have found out that is more important than actually standing. If you stand perfectly still, as you point out, it will not reregister. Also, you have to stand for at least a full minute in one hour and you only get credit once for that hour.
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