I've noticed since it's getting colder mine is having a more difficult time picking up my HR during exercise. I've found out if my wrist is moistened under the sensor it works quicker. If I don't moisten it, it will start working once I work up a sweat. Evidently when your skin is cold/dry it is more difficult to detect.
First thing to check is to make sure the band is snug enough. Then try restarting the watch. My heart rate sensor has failed to log heart during a couple of different workouts in the past. Restarting resolved the issue. Not sure if that will work since your problem is intermittent.