Yes, they only sell SRO when the show is sold out, or at least extremely close to selling out. I went a little over two weeks ago for a Saturday matinee, and when I first went to the box office just before noon, they said to come back in like 1.5 hours or so and see if they're sold put then. When I returned at about 1:30, the box office staff had to make a phone call to check, but fortunately, they allowed me to purchase the first SRO ticket for that performance.