Hi Jane,
You can try the old sandpaper trick. Using 220 to 80 grit sandpaper cut through the paper several times. If this doesn't work, it's time to send them to a professional sharpener.
You might consider purchasing a rotary cutter with a pinking or a wave blade such as the KAI 45MM Rotary Cutter. KAI's blades are made of very sharp high tungsten steel. I'm sure that the other major rotary manufacturers also offer these same blades too, and you can purchase them at a number of retailers . I know first hand that sales of pinking shears are down considerably due to the popularity rotary cutters.