Hi mysticfriend,
Thanks for using Apple Support Communities. Since you aren't getting credit for your movement, the first and simplest step would be to restart. That can help with many issues. We'd recommend restarting both the Apple Watch and iPhone. Reference the articles below if needed.
Restart Apple Watch - Apple Support
Restart your iPhone - Apple Support
If the same continues, we'd recommend unpairing and then pairing the Apple Watch again. That will refresh the connection between your devices. The following guidance is from Unpair and erase your Apple Watch - Apple Support.
"How to unpair and erase your Apple Watch if you have your iPhone
- Keep your Apple Watch and iPhone close together as you unpair them.
- Open the Apple Watch app on your iPhone.
- Go to the My Watch tab and tap All Watches.
- Tap the info button
next to the watch that you want to unpair. - Tap Unpair Apple Watch.
- Tap Unpair [your Apple Watch name].
- For GPS + Cellular models, choose whether to keep or remove your cellular plan.
- If you want to pair your Apple Watch and iPhone again, keep your plan.
- If you don't want to pair your Apple Watch and iPhone again, remove your plan. If you aren't going to pair with a different watch or iPhone, you might need to contact your carrier to cancel your cellular subscription.
8. Type your Apple ID password to disable Activation Lock, then tap Unpair."
Then pair with your iPhone and check your monthly challenge to see if the same occurs.
Take care.