Are you looking for weekdays or weekends? I believe Fantasmic is only on weekends in September.
River Belle Terrace has very limited seating for the premium package. Because of that, it sells out within a minute or two.
Blue Bayou tends to sell out before River Belle which tends to sell out before Rancho del Zocalo. For all, the first show sells out before the second show. I don't think it has to do with available seating for the show, but more about the capacity of the restaurant.
I checked around 7 am PST today and there were reservations open for both RB and RDZ for a party of two on Friday, September 20. Now, at close to 8 am, there is only one reservation for RDZ.