I really have no idea how to ask about this.

So I was trying to make an art brush when this happened. I've spent too long trying to figure out how to type my problem out on Google. It appears that my lines would fill when the stroke is not straight. Note: I am a bit new with Illustrator. I've looked at the default brushes but they appear to have the same settings. Any help would make my day! Sorry that I can't be more specific.
