Just came on to second this. DF and I took two five-day trips to the park this summer. With the exception of July 4 documented on this thread, we had no problem making reservations for any day within the 30-day advance window or making reservations for additional days the moment we were scanned into either one of the parks.
The July 4 blockout I overcame at 10 p.m. the night before, literally right after fireworks. I had already secured a res for CA two days prior--which was what was so strange about the July 4 blockout for DL being lifted for just 15 minutes July 3.
It's still a mystery how Disney is determining capacity for these FP "reservation only" days, but I had been refreshing the calendar in the app every day for 30 days out. CA was ALWAYS available July 4. Only DL was blocked out, and it was blocked out consistently, right up to the day before.