You were almost certainly not able to book it because it was not available. It doesn't matter that it showed available. It probably was available for a few seconds, but then someone clicked on through and got it before you did.
I can tell you that ALL the Riviera Studios have been booking out within micro-seconds of the start time at 8 AM Eastern.
The Tower Studios aren't even available, since they all booked out before the 7 month window.
The Standard View Studios are sometimes available at 8 AM Eastern, but on about half the days for the next several weeks, they too are already booked out.
The Preferred View Studios have been, very briefly, available every morning. But they are gone in seconds. I had to try over 14 times on many days to finally hook one.
People are walking all the Studios, so, sometimes later on, one will appear briefly, but then people grab it very quickly.
I wonder, if you put in a Waitlist Request, if Disney DOES by chance Declare more rooms in the next few months, is it possible that some of those might go immediately to the Waitlist people?