Can't make somebody like you. If they are not interested you have to be able to accept that they don't want to be with you. You think you should ask her out or write a love letter. Remember if she does't like you it isn't you fault. You're just not her type.
You can't make someone like you. You have to show them what's to like! If you don't want to ask her out just yet you can always try to get closer to her. Drop a few hints. If she gets to know you there's a pretty good chance she'll like you so ask her out then maybe. Be careful though don't get friend zoned. Good luck!