-You shouldn't have suggested lunch. Lunch is an opportunity to get friend-zoned. Ask if she wants to have dinner or something where you can interact together in a more romantic setting.
-Maybe she's nervous. Suggest that you both go out with some friends for drinks one day. It gives you both the opportunity to hang out together, but still have some security with having friends there. Another huge way in is if her friends like you. At that age she probably cares more about what her friends think of you than how she actually feels. If her friends give you the seal of approval then you're usually in.
-Pick up on signals- Does she play with her hair when talking to you? If you are talking in a group does she face you or angle her feet towards you? Does she turn away and laugh a lot when you talk to her? All those things suggest that she is interested in you and/or that shes nervous when talking to you (a good thing).
-Any sort of physical contact is good. It can be as simple as a high five or lightly pushing her arm when you make a joke. Don't grope her or do anything inappropriate, but physical contact between you both is good.
-Maybe she really did have to work? At this point you have nothing to lose. Don't text her every hour on the hour, but if you haven't spoken in a few days, call her up and ask her out again. Be direct and forward. Don't be pushy or annoying.
-If you're texting her- Use smiley faces. I'm not kidding when I say this, but it makes a huge difference. Joke a lot and use the
or :P afterwards. Another thing I've picked up on is at night if you're texting, "night" and "good night" are completely different. It sounds stupid, but I always say "good night" if I'm talking with a girl. Usually, when I struck out with a girl it was met with "nite" or something short. The best thing that can happen is that she thinks about you before she goes to bed.
-Don't play games. You're not in high school anymore. If you want to hang out with her, then ask her. Don't be pushy, but be upfront and confident.
-Simply put: confidence, humor, and directness go a long way. Don't second guess yourself, don't try to show off and ultimately be yourself. If you want it to go anywhere she's eventually going to figure out who the real you is. It's better she find out now rather than later.