The title on the offer does not match a listed profession, or the duties do not match the title. Officers assess the work actually described, so a supervisory or hybrid title can move a qualified professional outside the category.
The title is the most common point of failure
Hypothetical example: a Sainte-Brigitte-de-Laval applicant with a science degree is offered a position called laboratory supervisor. The Appendix 2 list has no such entry. If the duties are genuinely those of a medical laboratory technologist who also coordinates a shift, the letter should describe the technologist work and the profession claimed; if the duties are mainly administrative management of staff and budgets, the role may fit no TN profession at all, however well qualified the applicant.
The employer's willingness to change the title is not the answer; the officer examines what the person will do. A second frequent failure is the qualification rule: an applicant holds the right kind of degree but in a different field from the one the entry names, or holds a provincial licence where the entry requires a degree. Each of these is visible in the offer and the credentials before anyone travels, and each is cheaper to fix by rewriting the plan than by arguing at the border.