If she's too comfortable with you = friendzone
The key point is not making it AWKWARD but keep the SEXUAL TENSION up.
You want her to feel TENSION, because she will get NERVOUS, and NERVOUS will lead to her HEART BEATING fast. At this point, she will ASSUME that she is attracted to you, or at least she believes that's what HER HEART telling her.
It's just a normal physiological process of human body, but her heartbeat and brain will tricked her into thinking it's love or attraction. Woman logic is a wonder isn't it
Use this as your advantage.
The second thing you need to fix is excessive smiling, we tend to use smiling to break tension and build rapport, it's not WRONG, it's perfect for social vibing. However, in attraction, like I said, you want the TENSION to be there.
Here is a funny way to improve, I did it before. DO NOT SMILE the whole conversation even if she tells you a joke, keep eye contact though. You will see how funny they will try to qualify themselves for your smile. When ppl dont laugh at your joke, you start to doubt yourself and wonder if you're actually funny, if your joke is lame, etc... Put this pressure on her.
This also put you into a mindset that you're the prize she needs to win, you will eventually realize you don't have to be the one talking smiling the whole time. Once you have all your fun with her, now spare the poor girl and give her your best smile.