Well, yes! Pizza Boxes are difficult to recycle using standard practices.
All pizza boxes are made of corrugated cardboard. While there are standard procedures to recycle cardboard, pizza boxes are a different story altogether.
The cardboard of pizza boxes absorbs a lot of items like oil, grease, cheese and other food items. Once soiled with these items, the fibers of the cardboard cannot be separated from them using standard procedures making recycling difficult.
During recycling, when the fibers of paper and cardboard are mixed with water, oil eventually separates from the fiber. Since the oil does not dissolve in water, it gets re-absorbed by the pulp. This may at times result in a lower quality fiber, or recycled paper with oil splotches or at times also ruin an entire batch of potentially reusable paper.