Are you sure that you are awake when you study? Do you know what you are doing? (Example: If someone asked you what you just did or read, would you be able to say it in your own words?) Some things I ask myself just to make sure I am not half-dead.
One way to approach this is ask yourself 'what does my teacher want', nice writing, organized notebooks, participation in class? Also, what are the students getting better grades doing that your not doing? Each teacher's different, give them what they want, not just doing things your way. This can help, really.
Ok, could be several things. One, you are not paying attention. Two, the teacher is not teaching right. Three, it doesn't matter how hard you stud; your grades reflect your work ethics and how hard you work, not how smart you are. Four, maybe you are doing your homework but you are not doing it correctly or not getting the correct answers. Try studying less, with breaks in between. Pay attention more in classes and ask for help. There is no such thing as stupid questions, just stupid people who ask questions. Hope this helps!