I've been trying to help an old friend of mine polish a script he's written. As is typical of a lot of scripts, it's the middle that has the problem--even though lots of things are happening, that part felt padded. It took me a little bit of time to figure out why: the wrong kind of obstacles.
The obstacles were what I call mechanical. Most of them related to finding a particular character. However, there was no effective ticking clock, so there was no urgency to finding that person and following the protagonist through the steps was not compelling even though the individual scenes were well written.
The obstacles that work are the ones that have an emotional impact. If there's a ticking clock, the charater's emotion is fear; if the steps reveal that the situation is even worse than suspected, the protagonist's emotion might be despair; if they suggest there's a quick solution,the protagonist feels hope or joy (naturally since we're in the middle of the story, this will be short-lived) and so on. If you've made us care about the protagonist, we will share in these emotions.
Looking at the kinds of obstacles you create for your protagonist is a good exercise in making sure that your story moves forward and avoids the dreaded saggy middle.
(Want more methods? Check out www.timetowrite.com and, for screenwriters, www.ScreenWritingSuccess.com)