Shows don't usually put tix on sale for opening night at all...until they figure out how many they need for producers, family members and such. Once that is figured out, remaining tix often are released, but you never know how many or when.
Same thing happens with "premium seating" at a certain time premium seats that haven't sold at those prices are often reduced to regular priced tix and released.
House seats are often held until the last minute too.
So...shows that may appear to be sold out now, may actually release tix next week. So it never hurts to keep trying a specific date. (Thats why you will hear about people still getting tix to see Mormon, for example.)
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