The Bottom Of My Graphic 45 Red Rubber Stamps Turned White

I have been cleaning some unused stamps from graphic 45. I had never inked them and was storing them in one of Tim's zipped stamp notebooks. They had turned white on the rubber bottom part of each stamp. It happened to four different sets. One set also had two stamps that had turned black and smelled. I was able to clean them all up using a stamp cleaner/conditioner, but one of the stamps that had turned black still smells a bit even after washing it. Has anyone ever had this happen to their stamps? What can I do to keep it from happening again?