Keep checking in periodically with dining services, cancellations will happen.
I had good luck on our last trip in Nov. getting a last minute CM's ps. We decided to try CM's just a week and half b/4 we were to leave and when I initially called the meal was all booked up. But two days before we were scheduled to leave and a week b/4 the actual meal I could actually choose what time I wanted as there had been so many cancellations of ps holders!
Even when you arrive at WDW, call dining services then to check, I bet a better time for you will open up.
They have very few PS's it seems at Chef Mickey's..but lots of extra tables. We just go at 4:45 and we have always gotten in, and with plenty of tables to spare..in fact in May, it was dead, yet we couldn't get an early PS. Last trip, we went at 6:00, put our name in and rode the monrail once around. When we got back, we got in...and there were still a few empty tables.