I don't think Disney is holding any Magic Key reservations back, but as guests cancel, those go back into the pool. I think you just have to keep checking and hope to get lucky. We decided to add a day to our trip and when we first looked (I think it was around 5:00 pm), there was nothing available for the next day. Later that evening (closer to 9:00 pm), there was availability. So, I think it's just a matter of persistence and luck. My guess is that because guests have until 11:59 pm the day before to cancel, there is a fair amount of movement the day before.