Apologies, thought it was the other way around.
Having looked at the other episodes, the only answer I can give is the same one I gave earlier in this thread; for whatever reason, the film studio/content provider has decided that those episodes are only (currently) licenced for viewing via streaming, not download.
I agree it is odd that a couple of episodes from the middle of the series are the exceptions to that, but there could be specific reasons why. Ultimately, we don't know the reason why, it just is that way.
In time, the rights may change and Sky will again be able to make all episodes available to download. Unfortunately, when or if that will be, no-one can say.