redwood trees

Two years ago five redwood trees were planted that were each 6 feet tall. Four of the Redwood trees look healthy with lots of new green growth. Unfortunately, one tree is not growing nearly as well as the others. It seems to be growing much slower than the others and it has very little new green growth. All five trees get the same amount of water and get the same amount of sun. What are some possible reasons that the one redwood tree is not growing well and how can it be treated?