Starting a few years ago now Disney started making it more difficult to just drop or add days to a reservation. Now, instead of simply dropping or adding the days, the new dates you want to book have to be available in their system under the discount you want to use. The days you are dropping don't count. For example, if you have Oct 21-27 booked and want to change it to the 21-16, Disney has to have the 21-26th available under the discount (or rack rate if that is the case) you want to use prior to you dropping your dates. Same for adding. If the days you want to add are available, but the entire time including the added days isn't available, they might not be able to give you the longer stay unless you want a second reservation for the added days. I have heard some people have luck getting guest services involved and overriding this, but that the reservation CM cannot do this. It is supposedly to avoid rogue single nights or short openings they have trouble booking. It makes it a lot harder to change dates after you have booked, and I personally hate it. Did I explain that well enough? It can be hard to figure out, because it feels like the days ARE available, since you already have them, you just want to drop a night or 2. But they don't look at it that way.