I check the Peanuts website just about every day in the hope of finding a strip that I haven't seen before, and this past week I hit the proverbial jackpot: a great week-long story that I'd never seen before and, as far as I know, was never included in and Peanuts collection. (But it's not from 1960, as the site mistakenly says; it's from 1969.) It starts here and climaxes with the strip they ran today. One of the interesting things about the sequence is that Linus sort of becomes a surrogate for the audience, screaming at Charlie Brown to finally stop standing there and doing something, and becoming more and more frustrated until finally exploding with rage at Charlie Brown's wishy-washiness.
As to why it was never collected, it's probably because Schulz soon abandoned the idea of the Little Red-Haired Girl moving away, and moved her back without any explanation as to why or when she came back.