When did my pillow stop plumping?

Last night I slept well at the Hilton, where I was a guest. The cushion gave me instant bliss. It shaped my head and neck and gave me a good night's sleep. The entire ordeal has left me puzzled about the issue of my flat pillowcases. I have to replace my pillows annually because of how rapidly they flatten out. I wanted to know what made the pillow...