Pillows are going to lose their fluff and become somewhat flatter as you sleep on them. This can't be helped, as it's a natural effect of the pressure of being under your head for hours at a time. \
However, you can very easily fluff your pillows back up by placing them in the dryer for a few minutes. Pillowcases do not slide off fluffy pillows so easily because the pillow has more volume to it. \