I find some to be great producers and others not...Sometime they do just die...My breeder friend confirms that and depending on your numbers will also determine how much and fast you notice the loss...I've had some feeders pass trying to breed them, but my Dumbo are doing well (except for the Blues, they are WEAK genetic rats and I'm not weaning them out)...Just make sure your husbandry is spot on and keep on going...