If your Fruity Pebbles bars are freshly made and they're hard, it may be because they were pressed into the pan too firmly. While you want the cereal to stick to the marshmallows to form bars, you don't want to pack them into the baking dish or they will be hard to bite into! Be sure to just lightly press them into the pan. Overcooking the marshmallows or melting them at too high of a heat may also result in hardened cereal treats. If you made these Fruity Pebbles bars a few days ago, they may be stale. If you made them more recently, your airtight container may not have been well-sealed.