The Apple Watch will record exercise once the heart rate gets up to a certain rate.
For example, if I go for a walk and go slower than 3 mph (20 minutes per mile), I’ll get no exercise minutes.
If I go faster then 18 minutes per mile, it will count - in between is kind of a crapshoot.
So next time you walk - try picking up the pace a bit and monitor the exercise minutes. The faster you walk, the greater the chance that it will be counted as an exercise minute. A little trial and error should help you figure it out.
As you get more fit you’ll need to walk a bit faster to get the heart rate up.