For titles that haven't been released on DVD yet or for which the release date is unknown, we offer the option to save them in the "Saved" section of your Queue.
Sometimes a title will disappear from your regular queue and show up in your Saved section. This could be because it goes out of print, or all the copies get broken or missing, or whoever holds the rights to it stops dealing with Netflix, etc. There are all sorts of reasons. Don't give up. I have seen them come out with a new edition shortly after.