There are numerous reasons why a book may have a "Stacks" status in the catalogue, but not actually be on the shelf. Locate the sorting shelves on the floor, and check to see if the item is there. If not, you will have to check back at least one other time, and if the book is still not where it should be, go to the Loans Services desk, and request a "Search". The results will be available within 48 hours.
In Robarts sorting shelves are in middle of the yellow section on each stack floor.
At Gerstein: sorting shelves are near the elevator on level 1 and 2 below: as you face the elevator, look to the right to find the sorting shelves. The library asks that you check the shelves at least twice before placing a search at the Loans Services desk on the main floor.
This will vary from library to library.