Some papers have more difficulty in flattening our then others. This is especially true during the cooler months but can happen during any time of the year. The papers the prints are produced on are originally in rolls and the prints are printed to these rolls. I most cases we will ship prints up to a certain size flat so that they have had time to loose their curl and flatten out however this is not a guarantee. Thicker or heavier weight papers may still need more time to flatten out. We have a blog posting on our site which discusses this in more detail as well as some solutions.