Automatic workout detection stopped working

Was really happy the first couple of times the Automatic workout function worked when I forgot to start my outdoor walk but the past couple of walks are not getting detected as workouts. My Watch alerts me that my activity ring closed whilst I'm walking the dogs but no longer detects it as a workout. Heartrate goes up, registers my steps, I presume GPS is detecting that I am moving.

Power saving mode is off. Start workout reminder is on. I even modified the metrics to only include distance, heart rate, average pace and duration.

Is there a limit on duration? - I walk for 30-40 mins, averaging 1km every 12-13 mins. heartrate rise to 110-120. Not sure what else I have to do to get this working again.

Health app is short changing me on duration for exercise because of this which is really annoying.

Hope this is something simple that I have overlook because generally happy with my Apple products.