50's is often one you have to book for dinner times 180 days out or soon thereafter. (In my experience) Just keep trying up until and on your desired dates.
did you check on lunch times? I have noticed that the dinner times book up fast but often lunch times are pretty readily available. Its the same menu either time so there is no advantage to an actually dinner reservation in regards to that.
It's one of the most popular restaurants in The Studios, if not the entire property. It's tough to get ADR's there depending on your trip dates, times and size of you party. We're eating there today, though in the Tune In Lounge portion of the restaurant. One of our favourites as well!