From my slushpile and the class I just taught, as well as this post by Les Edgerton, I've surmised that the biggest issue that newer writers struggle with is CONFLICT.
Conflict drives Story. In fact, I'd go right out there and say Conflict IS Story. And that conflict essentially amounts to the events, people, and character traits that stand between a Protagonist and his or her goal.
The best stories state that goal very early on; hell, even in the first line, maybe. Protag wants X. Will they get it? amounts to the story question. (The easiest way to think of this is in a murder mystery: the detective wants to find the murderer.) Event number one establishes what our protag wants (inciting incident). Event two is something that keeps him from it. And so on, until the end, when Protag finally overcomes all the obstacles to achieve what they want (or not).
The best stories have conflict at every turn. Absolutely NOTHING works out for our poor protagonist. (Think of the hotel suite waking scene in THE HANGOVER as a prime example.)
If someone throws up, he should get it on himself. If a girl walks down a lonely alley, something bad should be lurking. If a boy wants to meet a girl, someone had better step between them. If a character is scared of spiders, its best for your story if one drops on the top of her head by the second act.
Exercise: go through your latest story, chapter, whatever, and see if anything works out for your character. Nine times out of ten (actually, closer to ten times out of ten) the story will benefit from it not working out.