Any idea why my fitbit lost my workout?

I've had a Charge HR for just over 3 weeks. I did two workout DVD's tonight, one was 40 minutes & at the end of it my calorie burn was about 1600. Partway through the second disc I checked and it was just over 1700 cals burned. When I checked again a bit later, it was at 1577. It didn't record any of the active minutes or calories burned. Any idea why?