I was at YC a few weeks ago and the front desk manager happened to mention that the resort was fully booked through the end of July (at that point), so it could just be a popularity thing.
But...as of last week, I had 5 days booked at YC in August under the summer discount. I wanted to add a 6th night. I called WDW a few times and they kept telling me it wasn't possible, that the resort was fully booked. I didn't understand, because when I searched for just that one night on the website, it came up as available (rack rate and summer discount).
So I booked the one extra night online, with the summer discount, then called WDW to ask if there was a way to combine the reservations. No one could do this, for some reason. After speaking to 5 agents I was told that I couldn't book/link all 6 nights together and still get the summer discount. I could cancel the 2 reservations and then re-book the stay for the whole 6 nights, but I'd pay rack rate. They kept pushing BC on me, saying the discount was available for the whole 6 nights there.
Long story short, if you don't mind the slight possibility of changing rooms, maybe try breaking the trip into chunks and see if you can get the dates you want!