If there is no secretary or the secretary does not keep the advisor's schedule, the student should learn from the advisor when office hours are held or when the advisor can be reached to make an appointment. Advisors will be out of town from time to time, even on Add and Drop Dates. While we encourage advisors to be available at such times, students should never assume that their advisor will be in. Always make an appointment about a week ahead of time. Students should not be overly upset at being 'stood up' occasionally.
Professors really can be absent-minded, and plans do change.
Keep making appointments. In our experience, students are prime offenders in this regard, no-showing or being very late as much as a third of the time. Try not to forget your appointments. All advisors are very busy. However advising is an important part of their job, and one they should take seriously.