To increase fleece volume you need to increase food intake and make sure they are getting all the correct minerals that they need. You can see this clearly if you compare two flocks of the same breed, one kept on a mountain and one kept on a lowland farm. The lowland animals will generate a much bigger fleece purely because they have the excess energy available to do so compared to the mountain sheep.
Also, some individual animals will have short or long fleece genetics (within the same breed). Keep only those that produce the fleeces that you want in order to enhance the traits you desire.