and are you sure you clicked on the button to change them, not just changed the numbers, but also affirmed?
Normally if you do so, you can always check at the top of the page. It says the current prices and the future prices between brackets. There you can see if you succesfully changed your prices for next month...
Hope this helps for the future.
If you did this and are sure you saw that your next months prices where the ones you entered, and now they are not right, you should post in the bugs bugs bugs forum.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.