Hi Dave
The exercise ring (part of the Activity app) requires activity above a "brisk walk" - with a sustained increase in heart rate for at least 1 minute - in order to give credit. What constitutes a brisk walk depends on your personal information.
If you are not using the Workout app, it's important for your arm to swing naturally as you walk or run, because the Activity app relies on arm motion and the accelerometer to track movement.
To get the best results from fitness activity, ensure that you have Wrist Detection turned on (Apple Watch iPhone app > General > Wrist Detection toggle).
You can record walks as "workouts": use the Workout app (eg Outdoor Walk activity) and start and end your activity accordingly (force press the screen to pause or end a workout, once started). This will also monitor your heart rate continuously during the workout.
If you also take your iPhone along on a few outdoor walks (with the appropriate settings selected - see below), you can calibrate your Apple Watch. This will enable it to more accurately recognise activity and provide more accurate estimates, including Move estimations in the Activity app.
Also ensure that Apple Watch is fitted snugly to your wrist so that the sensor can measure your heart rate - and also so that it doesn't accidentally loosen to such an extent that the watch thinks it has been taken off. It may help to tighten your watch band before a walk and loosen it again afterwards.
More info:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Use the Activity app on your Apple Watch - Apple Support
Use the Workout app on Apple Watch - Apple Support