Some don't always show up correctly in the libraries. I bought the 3 in one package deal for star wars 1-3 and now it no longer shows up in my regular library list, and it also won't show up in the global catalog of all RiffTrax offerings. Nor do they show up individually.
The only place I can find them is if I look at my purchase library (the list that shows your purchases in order you made each purchases). I forget it every time I want to watch them and I have to go hunting all overt the place before I figure it out again (in fairness thats more on me, narcotics not so good for the memory).