1. You didn't negotiate well when you were hired. There's a lot of variation in whether and how people negotiate salary when they get a job offer. Some accept on the spot, others push for a little more money, and others push for a lot more—and some of them get it. (Unfortunately, asking for more after you're in the position probably won't close the gap; it's never as easy to negotiate after you've accepted the job.)
2. You haven't made sure your accomplishments on the job are visible. You could be doing the best work in the company, but if your manager doesn't know about it, your salary probably won't reflect it. Don't be shy about sharing accomplishments with your manager, whether it's glowing feedback from a customer, a tricky problem that you solved before it blew up, or a cost-saving measure you implemented.
3. You haven't pushed for a bigger raise. Many people just wait for their employer to reward them with a higher salary and simply accept what they're given, even when the amount disappoints them. Sometimes getting more money is as simple as asking for it—but you do need to ask.
4. You think doing your job adequately is enough. Doing a merely adequate job isn't enough to merit a significant salary increase. Big raises go to people who go well beyond the minimum expected. Speaking of which…
5. You haven't taken on enough responsibility. Big raises don't just get handed out because another year has gone by while you do the same work. A raise is recognition that you're now contributing at a significantly higher level than when your salary was last set. A raise says "your work is now worth more to us." So you need to make sure that's true before you make your pitch.
6. You're difficult to deal with. If you're defensive, a prima donna, negative, a regular complainer, or if you're otherwise difficult to work with, you might not be frustrating enough to fire, but no one is going to go out of their way to keep you, either, and that includes increasing your compensation.
7. Your skills aren't worth as much in today's market as you think they are. If you don't know what your experience and skills are bringing in today's job market, you might drastically overestimate what your skill set can command. Do some research on industry norms for your particular work in your geographic area and see where your salary falls relative to those markers.
8. You're not as valued as you think you are. Many people overestimate their own performance and their own value to their company—assuming the company would fall apart if they left, when in fact the company would continue on just fine. Pretend that you're your own manager and ask what about your performance would really impress you, or what about you your manager should be upset to lose if you left. If you can't come up with much, assume that your manager can't either.
Ultimately, if you don't like your salary, talk to your boss about what you would need to accomplish to earn a raise. That conversation could lead to valuable feedback, which might help you discover a solid path to the salary you want. And if your boss won't budge, you can always go out and see what other offers the world has for you. You might find one you like better—or you might decide that you'd rather stay put.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.