Please make sure to place the extension (Part “C”) directly on top of the attached piece. Use the screw (Part “E”) to screw it through the already mounted piece into the extension, to fix it up.
You can also refer to the manual from step 5b on. As shown in the picture, you have to screw it from the back of the mounting piece. Also remember to loosen the big silver screw in the middle of the extension, to fit it over the shorter mounting piece with the silver One For All logo on it.