As I said before, wrap the threaded rods with foil or some metal HVAC tape to make them a larger diameter than they are. That'll make it easier for the coupler to clamp onto the threaded rod. It may seem like it, but the couplers are not threaded. They are meant to clamp onto smooth shafts. Loosen the screw that closes the clamp on the threaded rod. If you have a thin screwdriver, try wedging it into the clamp slit and pry it open some. Reinsert the threaded rod after applying the foil or tape to it. Tighten up the screw that closes the clamp onto the threaded rod. If the threaded rod still spins, you'll need to try again, adding more foil or tape in order to improve the grip between the coupler and the threaded rod.

Again, there are different couplers that grab the threaded rods better, but you should be able to make these work.