PBL without engagement is like music without notes.  How to avoid this dilemma?  Write a driving question that is open-ended, intriguing, relevant, and complex.  Develop community connections by inviting experts into the classroom either digitally or in person.  Ensure that student work is real work with a real application, and that they present to a real audience comprised of members from beyond the classroom walls.  But most of all, remember that what is engaging to the adult may not be what is engaging to the student.