Here is a post from Johnathan_uk from two years ago that might help:
"If you have not yet done so, it may help to update your iPhone and Apple Watch to the latest software, starting with your iPhone:
To credit progress towards your daily Exercise goal, the Activity app aims to identify activity that equals or exceeds the intensity of a brisk walk. This requirement applies both during general daily wear and when recording workouts via the Workout app. Exercise credit is awarded for each minute during which this requirement is met.
The definition of a brisk walk varies from person to person and depends on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
Be sure to record your workouts via the Workout app, choosing the activity type that most closely matches your workout. This enables Apple Watch to use the most appropriate sensors and data sources when estimating the results, including using tailored calorie calculations for preset activity types.
During general daily wear and when using the Workout app to record workouts when GPS data is either not relevant or not available (eg Indoor Run, Elliptical), the Activity app and Workout app track your arm motion (measured by the accelerometer) when estimating results including calories and progress towards the Exercise goal.
For the best results, allow the arm on which you are wearing your watch to swing naturally as you move around and during workouts (where relevant to the activity type).
Calibrating your watch can improve the accuracy of estimations made by Activity app and Workout app. More information and instructions for calibrating your watch are available via the link below. The article also includes instructions for resetting existing calibration data and starting afresh. Resetting your calibration data will not erase your Activity history."