1. Character not strong enough.
By this I mean the character does not grab the reader's attention. There is not enough in the character to make the reader want to have their attention grabbed! A reader needs to either identify with the character and so follow the story to see if all works out in the end, or be very happy to see your character get their comeuppance and again follow the story through to find out if they do! (I remember wanting to cheer when Molly Weasley in Harry Potter "dealt with" Bellatrix Lestrange - and sorry but I refuse to accept that's a plot spoiler now! The point is the characters or Molly and Bellatrix grabbed my attention. One I wanted to succeed, the other not to and so I was hooked).
2. Story not strong enough.
There isn't enough conflict or drama to warrant there being a story! Things have got to happen in a story and sometimes those things are not necessarily dramatic but they are everything to your characters. The reader needs to find out how your people will react if things do or don't turn out well in the end. Is there enough in your story to make readers want to find out what happens next? It can be a good test to step aside from your work for a bit and come back to it as a reader would. Is your story the kind you would eagerly pick up from a book shop because its opening lines and its blurb grip you?
3. Characters not distinguishable from each other.
Each major character has to have their own voice. A reader should be able to tell who is who. If they can't confusion sets in and nobody will read a story like that.