Hi there Davescadds,
Thanks for connecting with us in Apple Support Communities! We understand you're not seeing walking asymmetry data on your iPhone 8. We're glad you're here, and we'll point you in the right direction to get this sorted out.
To start, you'll want to make sure your iPhone is running the latest software version, which is iOS 14.4.2. You can check which version is currently running on your iPhone in Settings > General > About. If needed, learn more about updating your iPhone from this support article:
Update your iPhone, iPad, or iPod touch
If your iPhone is updated to iOS 14.4.2, and this behavior continues, you'll want to connect with Apple Support for more help. This link will help you get started:
Contact Apple Support
Cheers.