Premium Coins decay when your Premium Account isn't active - that is, if you don't currently have any Premium Time redeemed. You will lose 0.3% of your Premium Points every hour when your Premium Account isn't active.
You can see that your premium account is not active right away, because your premium display in the top left corner of the screen will show a red "Activate!" text.
If you buy premium time, don't forget to activate it by using the "Activate Premium Time" button in the Premium Status window! Select the amount you want to redeem at the bottom, and hit the "Redeem" button: