You don't say what resort you're going to, but I know at CSS you can't make dinner reservations until you check in and have a room/suite number, but you can make spa appointments in advance.
You have to wait until at the resort and then book your reservation at the concierge desk for dinner. That way guests already at the resort can still get a reservation as well. Especially if they tried a restaurant and wanted a another meal there before leaving or just didn't know that the fancier restaurants required reservations. This is the only fair way to do this so all guests get a chance. I am not sure about the spa.