For anyone still facing the issue of not being able to make park reservations for all of the days for your trip (and you aren't an AP holder), there is another flavor of the problem:
Mickey's Not So Scary Halloween Tickets.
I found that once I made a park reservation for the day I originally had
MNSSHP tickets for, it would not longer let me make any reservations for the other days of my trip and I got the "One or more parks are not available for Disney Park Pass selections" message for every day on the calendar. What I did to solve this: canceled the park reservation for the former MNSSHP day, made reservations for every day except that day, and made the reservation for that day last. It worked!
Hope this helps someone else who has been struggling getting their reservations!!
(I can't take full credit for finding this solution -- someone else found it in a Facebook group.)