*Gaps in knowledge. Gaps in knowledge are easy to remedy, because all it takes is reading more. The less you know about something, in fact, the faster the progress. It is much harder to learn something new about Lorca, for me. I do learn new things all the time, but I have to wade through things I already know in order to get there.
*Bad writing. If you aren't a good writer, you can improve. A lot of it is just caring about it. Once again, reading other people's prose critically and emulating or avoiding things you like or dislike.
*Bad work / organizational habits. Here the problem is to break bad habits, and habits by definition are quasi-automatic and deep-seated. Here you must identify the flawed thinking that underlies the habit. For example, you might think: No use in writing between classes, I only have an hour.
*Bad thinking. Bad thinking is any kind of cognitive bias that holds you down. For example: this journal is out to get me, they never accept my articles. Or, I can't publish as much as other people, because nobody is interested in my field.
*Intelligence. Can you become more intelligent? I don't know. There is so much under your control, but at some level some people are just smarter than others. That's what you might be thinking.
Yet people who think that success is the result of inherent talent tend to do worse than people who think success is due to hard work. So you are better off just outworking other people rather than assuming you aren't smart enough. People write dumb books, too. There are productive scholars who aren't good, but who just don't know it.
There are also concrete ways of being smarter than you already are.
First: be around smart people, even if they make you feel dumb by comparison. Being the smartest person in the room a lot of the time does not make you smarter, but dumber, in the long run. Think of other people has the sharpening stone for your own intelligence. When I'm around dumb people I get dumber and more complacent about my own abilities. Read people's work who are smarter than you.
Secondly: work at it. You get smarter by doing smart work. It is like intellectual exercise.
Thirdly: be creative. Begin to paint or write music. Have strong secondary fields of interest.
Finally, be analytical about why you think you aren't smart. Is it because things take you longer to understand. What tasks cause you the most problems? Maybe you already have to be smart to figure those things out, but it might turn out that you are just less experienced doing certain kinds of intellectual tasks. If you are interested in doing them well, you will figure out how.