Old episodes not (yet) available for purchase

I’m working on as close-to-complete a collection of Car Talk as possible (I have 570 episodes), because my sons and I listen to them on long cross-country road trips (and because, sentimentally, my mom got me started listening 20+ years ago, and it reminds me of her and her laughter). So anyway, the point: I’ve noticed that an occasional episode in a sequence will just be left out of the catalog. It’ll have a listing in the “our show” pages, but only with a “best moment” audio clip. There’s no way to buy the episode from itunes or Amazon or Audible. Why is that? What’s keeping those episodes from being uploaded to itunes like all the rest?