Here are reasons the request button would not appear for a book or other item in OneSearch:

• You haven't signed in
• The book doesn't allow you to request it. Not all library materials may be requested. For instance, 2- and 4-hour reserve textbooks cannot be requested.
• The book may have copies that have been distinguished from one another (for instance, vol. 1 and vol. 2), and you need to click further to find the request button
• You aren't enrolled (with enrollment fees paid) in the current term
• You are trying to request a book very close to the end of the semester, or between semesters (requesting becomes possible about a week before the semester).