Check that you have the correct call number and location.
Confirm that the book is supposed to be on the shelf and not out on loan. If the status says 'recently returned' it may be in the returns room, on a sorting shelf or on a sorting trolley.
Ask at the service desk when the book was last borrowed. If no-one has used the book in a long time it might be missing.
Check with library staff that you're looking in the right place.