On a trip last January my buddy had a new cottage UG that was rated below the expected temps. He got into his hammock and I adjusted it for him. He was snug as a bug when he went to sleep. In the morning while he was still in the hammock I asked him how things went and he informed me that he nearly froze to death. I looked everything over and his cinch lines at both ends had slipped during the night. His movement had caused them to loosen. I tied slip knots into the lines to hold the position and on the next night he faired much better. Is it possible that your lines slip at night if you toss and turn and allow warm air to escape ?

Another possibility is body fuel. Are you eating a meal that is high enough in fats or calories to last you through the night ? Without fuel your body can't stay warm.