why is my garmin no longer counting walking as active calories?

I have had my forerunner 165 for about 3 months, and it has previously counted when I walked towards my active calories, however after walking for 30 mins on the treadmill today and walking around town for about an hour my active cals did not increase at all, even though I had accumulated around 9k steps. Does anyone know why it has suddenly stopped counting it towards my active calories??