Do you see any yellow on the surfaces you are trying to use for the 'Use quilt' cut? If so, it is not fully merged and that is job #1 ~ get the surfaces merged into a watertight quilt (if completely enclosing a solid) or fully merged quilt (if not surrounding but bounding and dividing sections of the solid). If the quilt does not encapsulate solid, then it must come to or past solid surface, at all places along the quilt. If you can't tell, then extend the edges of the trim quilt so that it sticks out past the solid surface you want to trim. You don't hurt anything with 'sloppy' trim surfaces. In fact, if the quilt is 'buried' inside the solid, it can't even be selected for use in the cut (unless it completely encloses solid). Clear as mud, huh? I've got the tutorials. Maybe you could refer to it, by chapter and exercise.
David Janes