You said they all appear to be 1.5 year olds right? Well, if thats the case you actually have a pretty decent age structure among the bucks on your property. The older, dominant bucks do of the breeding we all know that. This leaves the younger subordinate bucks at a disadvatage when it comes to the lady folk. They are basically beaten into submission and dont get as many opportunites to breed during the rut. This reults in their testicles not receeding as early, thus meaning they still have seamen in their testicles, thus meaning they are still able to breed. I hope this made sense?