Here is a possible answer to your problem that was given by an Apple specialist:
"Here are some additional troubleshooting steps to try in addition to what you have already done.
Check your Stand reminder settings:
On your iPhone, tap the Watch app. Tap the My Watch tab. Tap Activity.
Make sure Stand Reminders is enabled, and that you don't have them muted in the Today view.
Also, while in the My Watch tab go to General > Wrist Detection and confirm that this setting is enabled.
Try turning Fitness Tracking off in the Privacy setting of the My Watch tab.
Then restart your iPhone first, then your Apple Watch and turn Fitness Tracking back on.
Make sure Do not Disturb is turned off.
Apple Watch User Guide - Track daily activity
If the issue persists, you may need to unpair your Apple Watch and iPhone and pair them again. Back up your iPhone before doing this.
iCloud: Back up your iOS devices to iCloud
Unpair your Apple Watch and iPhone"