Reward her! Let her know how much you enjoy her giving you oral sex, and reward her with the same attention. Go down on her for a long time, and then ask her to reciprocate. If that's not the thing that gets her hot, ask her what is and make sure you do it frequently. This is a perfect opening for your asking her for something you like, after you've just spent a lot of energy pleasing her. Partners should be willing to do things that please their partners even if they don't necessarily flip the switch of both parties. Sex is a give and take. If you both are not giving and expect the other to give, you'll both need to go back to Webster’s and look up the definition of selfish. Discuss it, soothe her fears if they are there. Encourage her, and let her know how much you enjoy it when she does it. Most people get very aroused by their partner getting aroused, and if they do not, it could be a sign of some trouble. One sided sex is unfulfilling, and does not make for a lasting relationship. Be sensitive to triggers, though. There could be a lot of reasons she won't do it. She could have had a bad experience, been severely abused, or is simply not confident in her abilities in that area. If her reason is taste, get thin condoms and flavored creams. If it is a fear, it will take much more work and building of trust through communication and honesty to overcome. You can't make her enjoy it, but maybe you can help her in the general direction.