Im having trouble with my "sled dogs"!?

ok so i have sled dogs. 3 black labs. 2 15 week olds and a 1 year old. they understand " Hike" "Haw" and "Gee" and "Uimbai", and they can pull logs and me and my friend, Heather. But all of a sudden, they ran out of strength from their necks to their rumps. could it have been the harnesses? yes i live in Indiana. i know its weird, but we tought them. or in other terms, they KNEW. its weird, but it happened.