There are several reasons that students miss their self-imposed graduation deadlines; as it turns out, you need to carefully plan your course load each semester, to make sure it all adds up to the right number required to graduate. (It’s typically 120 credits for a Bachelor’s Degree).
If you’re working while enrolled, or need to transfer schools – you’ll have to continually re-evaluate the number of credits you need to finish, AND that those credits count toward your major. Even switching majors at the same school can affect your credits.
The New York Times online has an informative article that explains six reasons why students may not be able to graduate when they expect or intend to; chances are some of these may apply to you, and you weren’t aware. Check the list of red flags here.