You’re going too fast
Every author needs an editor, but authors need to know how to edit themselves too. Carefully evaluate what you’ve written and then go back to improve it. Are you saying everything you have to say? Is your argument clear? Are your characters compelling? Is your prose fun to read? Keep polishing and polishing until you’ve written the best manuscript that you can.
You’re going too slow
Your manuscript doesn’t have to be perfect the first time. You shouldn’t even think of a first draft as a “first draft.” Call it your “zero draft.” Don’t worry about structure, length, quality, or anything else. Just write. As you write, you’ll discover what you want to say as you go. Then go back and start crafting your work into a quality manuscript.
You’re not putting enough of yourself into the work
Great writers distill their experiences into their writing. Take a pen and some notebook paper with you everywhere you go. If someone in the street says something interesting, write it down. If a toddler sits in a stroller with a peculiarly composed posture, describe it. Describe the colors of the day, the temperatures, the sounds. Thread these descriptions into your writing.
Your writing lacks color
The key here is to write what you know. Set your stories in places that you’ve been—and then describe them. Start with a telling detail, and then layer on more details each time you return to the scene. The same goes for dialog. Characters should be intelligent and unique. Take note of people in your lives with distinctive speech patterns, and practice writing how they speak.
Your prose is awkward
Read what you’ve written aloud. If it doesn’t sound natural, it won’t read well either. Keep tinkering with what you’ve written until it’s elegant and concise.
Your writing is flat
Creating images in a reader’s head is the hardest thing for a writer to do. Read Dreaming by the Book by Elaine Scarry for advice on how to create a vivid, lithe, and sumptuous world for your readers.
Your writing isn’t professional
Rules are made to be broken, but publishers can tell when they’ve been broken out of ignorance. Find a good style guide and read through it a few times. Strunk and White is a good starting point, although many of its rules are outdated. If you’re unsure of something, check out the “Grammar Girl” online. The Language Log is another great online resource.
Your story isn’t believable
Truth is stranger than fiction, so even if it happened to you, a reader isn’t necessarily going to believe it when it happened to one of your characters. If your story is plot-driven (murders, kidnappings, etc.), it helps to write out all of the plot twists in an outline. Now imagine that you’re each of the characters. Are they taking the most logical steps, based on their personalities and what they know? If not, you’ve got to go back to the drawing board.
No one is going to be interested in what you wrote
Actually, this is a fake problem, an ugly doubt that causes needless writer’s block. If your story is unique and well-told, it has a great chance of catching on with an audience. Success seems obvious only after the fact. If you do your best, who knows what the future will bring. The world might be waiting for the story that only you can tell.