Very specific zipper question

I am repairing a couch cushion cover (Ikea product) whose zipper head snapped in the laundry. I wanted to try to replace only the head/pull because the zipper itself should be fine. It's a coil style zipper. On the back of the original zipper head was printed "5B". I did some googling and it seems like 5B indicates a #5 plastic moulded zipper. A #5 metal coil zipper should have said "5C". I bought a zipper repair kit that had heads for all types of #5 zippers: moulded plastic, metal teeth, and nylon coil. I tried each of the different types and they all resulted in sliding down the length of the zipper but not joining (zipping) the two sides together.

Does anyone have any experience with this? Do I need a smaller zipper head in order to press the two sides of the zipper together? Should I just give up and replace the whole dang thing?