Snoopy!!! had the same producer as You're a Good Man, Charlie Brown, Arthur Whitelaw. It used the same format: a bunch of sketches with dialogue taken directly from the Peanuts comic strip; songs based on situations from the strip; adults playing the children and Snoopy (and in this case, Woodstock too, a non-speaking role); minimal scenery. The composer-lyricist of You're a Good Man, Clark Gesner, didn't participate, but the songwriters hired for the show were quite a bit more talented than Gesner: composer Larry Grossman and lyricist Hal Hackady are best known to Broadway buffs for cult flops like Minnie's Boys and Goodtime Charley, and Grossman was one of the most talented melodists of his generation, though he never managed to associate himself with a genuine hit musical (he did win Emmys for writing songs for "The Muppet Show," though).
But You're a Good Man, Charlie Brown is a funny, unique and affecting show, and Snoopy!!! is kind of a mess. As far as I can see it got one big thing wrong and two little things wrong. The big problem was that a show focusing on Snoopy is not as emotionally satisfying as a show focusing on Charlie Brown. Snoopy is a more limited character than Charlie Brown, and he doesn't interact much with any of the characters except the even more limited Woodstock. So instead of the situations from You're a Good Man, Charlie Brown, which add up to a realistic "day in the life" of the central character, Snoopy!!! becomes a series of wacky sketches based on strips that weren't Schulz's best work anyway (Woodstock falls in love with a worm, and so forth).
Second, the songwriters made the very basic mistake of basing many of their songs directly on individual Schulz strips. There's a song based on the famous "looking at clouds" strip (the one where Charlie Brown is going to say he saw a ducky and a horsie, but changes his mind); a musicalized version of Snoopy's "dark and stormy night" novel; a song based on the strip where Charlie Brown wants to be called "Poor Sweet Baby"; the London version of the show has a song that takes one panel of Peppermint Patty saying "Hurry up, face!" and repeats that theme for three minutes.
None of these songs add anything to the strips they're based on, and therefore they sound like retreads of stuff we already know about the characters. Clark Gesner wasn't much of a craftsman as a composer or a lyricist, but he found exactly the right way to write for the characters: create songs and musical scenes based on broad themes connected with the strip (failure at baseball, trying to fly a kite) but without any direct quotes from or references to Schulz strips: the songs are where you go to expand upon the strips, not quote them the way they're quoted in all the dialogue.
Grossman and Hackady were good songwriters and they did come up with a few good songs (the musical scene "Edgar Allan Poe" is funny, and songs like "Don't Be Anything Less Than Anything You Can Be" are catchy albeit generic). It does seem, though, like they never really "got" the material the way Gesner did. Their attempts at writing emotional songs for the characters are downright embarrassing. Gesner, drawing on but not re-hashing the strip, found the perfect theme song for "Peanuts": "Happiness," a song about taking pleasure in the little everyday things in life. The same spot in Snoopy!!! is occupied by a maudlin song called "(If) Just One Person (Believes In You)," whose sentimentality and facile, unearned optimism goes against everything the strip stood for in its prime. Not to mention that the lyrics include an unintentionally funny line (at least, I hope it's unintentional), italics mine:
If just one person believes in you,
Deep enough and strong enough,
Believes in you,
Hard enough and long enough....
The song goes on to tell you that if just one person believes in you, soon many, many people will believe in you, and then "maybe even you will believe in you too." It's hard to get that message from Schulz's strip, where nobody believes in anybody, and least of all themselves.
So that's Snoopy!!!: more evidence that one-of-a-kind successes are hard to duplicate -- particularly if the creative team doesn't fully understand what went into the original success.