Sometimes as managers, supervisors, library directors, we communicate with our staff in polite, collaborative ways (after all, we want everyone to like us!), when we really need to be more direct and make it more clear that we are not simply making a suggestion but directing staff to do a certain thing.
If you find you are having trouble communicating, or if your staff thinks you are simply making suggestions, you might find How Can You Communicate With More Authority helpful.
Be very clear in your language and your statements. I will be interested to hear if you take this advice, and if it helps.