Remove the flexible white filter screen from underneath and then look at the bottom of the cylinder. You’ll see the bottom of the cylinder has four slots, and the metal frame has four metal tabs. You may be able to see if the metal tabs are not locked all the way into position.
Looking down on top of the ROK, the cylinder twists clockwise to lock. Undoing the top screw, releasing the levers and pulling the plunger out will probably make this easier. Also, this would be a good time to remove and inspect the cylinder for any obvious damage.
