That looks like mine. Read the manual, but...
Make sure the star washer on the bottom of the drum is flat up against the drum so the pins fit into the notches on the washer, then tighten the large nut up agsinst the washer. I assume you have a locking collar above the drum that you thread up tight against the bottom of the lower swaged stud on the forestay - these keep the drum, which is the turnbuckle, from loosening. That collar is under the aluminum foil clamp that is held to the steel drum with the 3 screws. Check that to make sure it is still tight.
Mine had been assembled incorrectly when it was first installed and the P.O. never fixed it. Luckily never lost the mast.