Sequels are weird. Everyone says they're harder to write than the first book, and everyone is right.
I can think of lots of great sequels, books (and movies) that matched or surpassed their originals: The Lord of the Rings, Terminator 2: Judgment Day, The Tombs of Atuan, The Illearth War. I can also think of plenty of sequels that disappointed me for one reason or another.
But until you write one, you don't really realize what you're getting into. You've got to extend the story that started in the first book, keeping things consistent yet taking the narrator and other characters into entirely new places. You've got to expand and enrich the world, revealing information that wasn't evident in the first book (but that appears to unfold naturally from what was provided in that book). You've got to find the right balance between filling in what came before (for new readers) and assuming a certain familiarity (for returning readers). You've got to up the stakes, complicate the situations, heighten the tension, and pretty much blow every reader's mind, new or returning.
Oh, yeah, and you've got to tell a good story while you're at it.
Maybe this is why my sequel has gone through three distinct phases. In the first phase, it was a mess. I didn't know what the heck I was doing, and I went in a direction that simply wasn't workable. I basically had to jettison 80% of that manuscript and try again almost from scratch.
In the second phase, things were moving in the right direction, but there were significant issues with plot and character that needed to be resolved, not to mention significant dead spots in the manuscript that needed to be removed. Maybe 40% of that manuscript ended up on the cutting room floor.
And now we're on to phase three, which contains the 20% from phase one and the 60% from phase two. But it's not as if I could just stick those two chunks together and add another 20% to produce the 100% I need to have a complete manuscript. No, I'm having to rethink the whole thing yet again--not only to reconcile the disparate parts but to transcend them, to produce something better than either of the previous two attempts.
Survival Colony 9 went through multiple revisions too. But though some of those revisions were substantial--chapters moved around, character relationships altered, and so forth--none was as substantial as each of the major revisions I've undertaken with book two.
I'm happy with where the manuscript is going right now (though I still have lots more to do). I not only hope but believe it'll be at least as strong a book as its predecessor.
But after I'm done, I'm going to stick with stand-alones for a while. YA Guy needs a break.