Yesterday, I replaced the fill valve in my toilet with a Fluidmaster 400LS, and everything in the installation was going fine until I came to the step that reads "Press down on top cap rotating top & arm clockwise to locked position." Until I did this, I could test water flow by turning on the water supply (covering the top of the valve with an upside-down cup so that water wouldn't spray everywhere.) Water is on and the fill valve is filling up the tank nicely. Then when I rotate the top and arm clockwise about 1/4 turn and it locks into position, no water will come out of the valve. When I turn the top and arm a 1/4 turn in the other direction to one of the other locking positions, I get the same result. I've turned the water on and off many times and fiddled with the cap and the locking position, but have had no luck in solving. I can't find any information about this problem online or in another forum post here. Has anyone experienced this before and have any tips on how to solve? Many thanks.