Spec for black water tank is 34 gal yet 21 fills it???

Sometimes stuff gets stuck down there. Reach your arm down the hole as far as you can. When down to the max feel around for anything chunky. Grab it and squeeze it between your fingers until it gets smaller then dump some water in it (After you pull your arm out of coarse}. Do this once a month for regular maintenance. Wash your hands before cooking. I do have a self help video available. Hope this helps