I've lost subadult snakes before for no apparent reason. Not all animals are "meant" to make it to old age. I doubt it's related to the pet store after all this time. They get their animals from the same "big time breeders" that people laud all the time on forums. Anytime a breeder sells a wholesale lot, the snakes might end up at a Petsmart or PetCo. Of the animals I have had pass away at subadult size for no apparent reason, all came from well-known breeders. It's just how life goes, unfortunately.