Avg Skin Temp Change not consistently logged after a night's sleep

The Avg Skin Temp Change seems to get logged on average 3 or 4 days out of 7. On the days it doesn't get logged the value is showing a null (a -- dash with no value). Does this mean there is no change or is there some reason why it doesn't get logged? Other measurements have been fine, albeit today Breathing Variations also had a null (-- dash) value.

I wear the monitor on either the outside of my arm or on the inside. Position seems to not impact measurements.