Two things could be going on, one relatively new. First, Exercise will only be tracked as EXERCISE if your heart rate is up enough for it to count as exercise. So a common mistake people make is thinking the Apple Watch is not tracking correctly. If the workout was correctly logged as 45 minutes, but your exercise ring only counted 15 of that, this is normal. Walk faster. ;)

But if the WORKOUT is not correctly logging the time, you may have the auto-pause turned on (and this is on by default). This will pause the workout if it thinks you are not working enough. If you don’t move your arm enough, it WILL pause the workout. For example, if you’re walking on a treadmill but you’re holding the rail, your watch may pause because it’s not detecting enough motion. I have had this happen on occasion on treadmill runs … even holding the watch up to check things for too long may auto-pause the workout.