(NOTE: There are always exceptions. And breaking "rules" is okay. But, presumably, you are writing commercial fiction and you suspect something is wrong. That's what these reasons are here for.)
1) You don't know the story. If you can't easily answer all these questions, get to work understanding the tale you want to tell.
- Who's the protagonist?
- What does he/she want/need?
- What are the stakes?
- What are the obstacles?
- What does success look like?
- What is the story question?
2) You started in the wrong place. Once upon a time, readers were patient. They'd give writers as much time as they wanted to establish the story world, and the background of the protagonist. Often, interest was carried by beautiful prose, usually written in third person omniscient. Forget about that. We have attention-deficit readers who need to be grabbed by the lapels and yanked along through the story.
- Do you have a hook in the first paragraph (or, better, in the title)?
- Is the protagonist (or an engaging character) introduced on the first page?
- Did you avoid the dreaded prologue?
- Do you have any big blocks of narration in the first ten pages?
- Is there a compelling problem or question (even if it is not the story question) introduced within the first two pages?
Remove all the description, narration, and backstory in the first 30-50 pages, and reread imagining yourself as a reader who is oriented, knowing the characters and the world. Does it work? If so, add back as little as is absolutely necessary for a reader who does need some orientation.
The slow start seems to be common for two reasons. The first is, a lot of drafting in the beginning is the author clearing his or her throat. What is this story and what does it sound like? You need those pages, but the reader does not.
The other reason for slow starts is a mistrust of the reader. They won't understand. They'll be lost. They need to know this. Mostly, this is nonsense. Take a look at contemporary books you love. Where do they take off? How much do they challenge readers? I remember when I started a Young Adult novel and wondered about expecting too much of the readers. I read the beginnings of ten successful novels. None of them led readers by the hand.
3) The story logic is a wreck. According to the South Park guys, for a good story, there is clear causation between beats. That is, scenes are connected by "therefore" or "but." (You can skip to 3:40) This is minimal, and it's amazing how many missing and unnecessary scenes are revealed using this simple analysis. The one caution is longer works, with subplots and B stories, will need to be recognized, with separate analyses for each story line.
For more detailed investigation of story logic, I recommend looking at Jeffrey Kitchen's "Writing Backwards" approach.
There is a fourth common reason why stories fail -- a weak or lousy ending. There are lots of ways to go wrong here. Rather than go into details here, if you suspect this is your problem, take a look at three posts I did on endings.
You can have a story with none of these problems that still is a 90-pound weakling. Help is on the way. Next week's Story Clinic blog will look at taking an anemic story and making it stronger.