Jacking is caused by too much work. That "overwork" maybe be one critter or 500 critters. It will all depend on the dog. All dogs will jack, you just have to know what standards you can accept and which ones you cannot. Go to the well too many times, too often and the sumbitchin thing will be dry. Give it some time to come back and some of them will and some wont. It all depends on the dog.