A lot of writers are good at beginnings–starting off with a bang. Then there are plenty of writers who are masters of endings, knowing how to tie things up in a way that is both dramatic and satisfying. I rarely come across anyone who favors the middle section of a story.
In my experience, this is the point where I usually tend to stall out. Not only is it the largest part of the story, rather than having one (or two) big dramatic events, you have to string together a series of events with tension running through the whole thing. A lot of writing instructors refer to the middle as the ‘try/fail’ cycle, a series of conflicts and obstacles that your protagonists must somehow overcome. The nature of these conflicts will determine the level of tension, provide character growth, and propel the story to its eventual end.