Why do some Netflix movies have an unknown availability date?

I know this has been asked before, but none of them have answered my questions! I'm a fan of the oldies, and so I like to be able to get them from Netflix to watch. But I've noticed that some movies aren't available at all, and I have to save them to the queue, rather than adding them. Is the movie just not available, i.e. they weren't able to find a DVD version of it? I'm signed up to have the movies mailed, but only have DVD. Are they available on Blu-Ray instead? If it's any help, one of the movies I wanted to rent was "The Thin Man Goes Home," but it wasn't available.