20 is very young, and the percentage of 20 year old women who don’t have orgasms during intercourse is pretty high (I don’t have the statistics handy).
My wife does not really enjoy receiving oral sex that much either, although she loves to give me head (lucky me). I would say the best thing you can do is love her as much as possible and give her the best sex you can. Don’t talk about orgasms during sex or immediately after, because that will make her feel pressure to have one, and that will make it more difficult for her. Enjoy the sex with her and make her feel that she is doing a good job pleasing you. Eventually she may become more relaxed and start having more orgasms.
Maybe you could suggest that she play with her clit during sex. It is a good way for her to start learning about her own bodies response. These discussions should be at dinner, not in bed.