I have tried every product available to dislodge my daughters surf board cover zip with no success but finally found a solution. The reason the zip was stuck was that sand had lodged between the teeth and the zipper and set like concrete. I used a thin needle to poke CAREFULLY around in the zipper and found lots of sand falling out. Eventually with a bit of wiggling the zipper worked. No use to use RP7 etc as the teeth are plastic.
Hope this helps someone.