My Apple Watch does not always record my outdoor walk on the exercise ring

Based on what I have read in the replies to Workouts Not Recorded by Exercise Ring - Apple Community, unless you use "Other" or "Yoga" you have to accomplish at least one of the following for a "minute of exercise" to be counted as an "exercise minute":

1. Heart rate higher than 130
2. Average pace lower than 18'10" per mile
3. Speed higher than 9.5 mph

I have also heard that according to Apple Support, for the activity ring to register your workout (for outdoor walk on exercise ring) you need to maintain a speed of approximately 3.3 mph or faster and increase your heart rate to approximately 75% of your High Heart Rate (from the iPhone Watch > Heart app). This definition of a "brisk walk" is to encourage you to put more effort into the walk, (For an Outdoor Cycle its is 9.8 MPH (or 16 KPH) and 75% of your High Heart Rate.) If you slow down or pause, the watch will stop giving you exercise credit, It will still measure your steps and calories burnt but will not give you any credit towards the exercise ring or workout app. A brisk walk speed is different for everybody as the watch takes into account your age, sex, weight, and heart rate which is why it is important to calibrate the watch and have the correct details set up in the Health app,

I can't vouch for any of those specific numbers, but I'm sure there is some kind of criteria. See the "Make sure that you earn Move and Exercise credit" section in:

Get the most accurate measurements using your Apple Watch - Apple Support

You should also make sure your watch is calibrated:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support