I'm doing an essay on it and my teacher says that my essay is two dimensional and lacks insight.

You probably will not like this answer but here goes. Your teacher is trying to get you to think more and look for the answer. Look for doesn't mean surveying us. You need to reread some passages and maybe have a discussion with classmates.

Macbeth, and Lady Macbeth, kill Duncan, in order for the Three Witches prophesy come true. Remember, the Witches called him not only "Thane of Glamis", "Thane of Cawdor" but tell him he will be "King hereafter". That was far too appealing to Macbeth, especially to Lady Macbeth. A real social climber, she was.

I thought that it was a BIG knife ;)

Yea I wrote an essay on how it was Macbeth's ambition and not the witches or his wife. I just want an idea of where I should go next. My teacher said I should consider other factors, like what's really going on in Macbeth's head, but I'm not really sure what she means...

Did you read ALL of Macbeth? He had so many thoughts running through his head. What was right, what was wrong, how he could get away with things, etc. The Witches played an integral role in the play - do not negeate them.

Keep reading - start from the beginning if you have to.