@Amy After reading your profile, a few things are totally clear, to me:
1. YOU can't have a system in which you must know in advance when the guests are arriving by so much in advance. That is not going to happen. It is asking too much from people who are 'on the road' traveling through a helter-skelter world. And I suspect from the type of clientele you primarily cater to.
2. Of course, you find it very frustrating to try to communicate with people who not understand punctuality nor can't extend themselves to see things from other's perspective. Your life and being appears to be the opposite.
3. So, a more 'dynamic' check-in system to let your guests in must happen; it will probably be probably a radical departure than the way your system is now.
Solutions:
1. An electronic system which allows them to check in on their own setting you immediately 'free'. Granted an impersonal opening, one that perhaps comes somewhat into conflict with the type of place you do offer and perhaps too impersonal, but perhaps you then see them 'when you get there'.
2. Have someone of similar ilk as you who can do the check-in for you, and then follow the personal touch, when/if you can.
3. ??? (Perhaps others can suggest other ways; I am at a total loss at the present.)
Just a thought.