When I wrote Secession Campaign I went through the whole process of breaking out a synopsis and creating an outline. It went through a lot of revisions and several significant changes before I published it. I went through the same process before I started on the sequel, taking advantage of a lot of the critique I got while working on Secession Campaign, not to mention that book provided a clear framework for the sequel.
Things were very different when I started on my 'steampunk' novel The 4TH Planet Problem. That developed more organically, started as short story structured out of faux newspaper articles and diary entries, became a medium length story centred around a single character and then grew into a novel. I thought I could do the same with working on the sequel, but this past weekend I had to bite the bullet and scrap what I've written so far which was a lot of words.
So what went wrong? Well first off I didn't wait to get feedback on the first book, so I was working a sequel to a book that only existed as a first draft. This was an issue, though it didn't seem like one because I expected the 4PP sequel to develop in the same organic way as the first, not needing a detailed structure. Added to that though this book was intended as a sequel the plot threads in 4PP didn't really create a guide for what direction I needed to go.
I had been struggling with the 4PP sequel for a few weeks now, chopping and changing things, with the writing going far more slowly than it was on the Secession sequel. It was only when I finally started getting some critique on 4PP that I finally had to accept it just wasn't going anywhere very interesting, added to which the feedback sparked new ideas that did give me a clear idea. So I've started breaking out a completely new plot and synopsis and I'm going to let it mature a bit before i go back to it.
Moral is if you can free form something it's great, but you should never count on it. if you're not sure about a piece your writing it probably is time to stop and think.