Take my Ben and Co. novels for example. I had actually intended for the second book's ending to close the whole series, and it was supposed to be over five books. The main character wasn't supposed to become a dragon until that final book, but instead, I put that for the first book. From that series of five books, I instead decided to make it just two books. But then, I once again decided to make it about more than five, and started writing a third one. It's being published chapter by chapter on Hubpages.
Depending how attached I am to a story, I tend to work on certain ones more often than others. I currently have one story, also being worked on in Hubpages, on hiatus because I'm more into the other stories. I do plan on getting back to it eventually. Just not at the moment.
I think I mentioned this in an earlier post, but I'm mostly attached to adventure, fantasy, sci-fi, and mystery stories for kids and young adults. Those are the kinds of stories I like to read, so those are the kinds of stories I wanted to write.