Firstly, don't shave your head. That's usually not a good idea for us special type o' boys.
Plus, from what I see of your picture, the hair looks fine.
Not passing can come from a lot of different variables.
Hair, facial structure, voice, body structure, clothing, mannerisms, speech, even the way that we walk.
You might want to take a look at all of these things individually and see what might lead people into misreading you.
Or the next time that you get falsely identified ask, "What makes you think I'm a girl?"