Hmm that doesn't seem so good.. I would normally say 'wait and don't put pressure on her', but 2 years is ages to be in a relationship so she should be comfortable enough with you do it. Assuming you have made it obvious that you'd rather like to receive oral sex, she must have some other issue with it. And since you're in a mature, long term relationship, I suggest you talk about it.
A lot of girls don't like blow jobs. They find it icky or uncomfortable or even insulting. I doubt it's many girls' favourite thing to do (though I have to say I love it). Or maybe she just isn't confident with doing it, and thinks she'll do it wrong. You really need to find out, but not make a huge deal about it, which could be tricky. I guess just mention it in a sensitive and vaguely subtle way, and definitely DON'T do it when you're being intimate, cos that'll put pressure on her and ruin the moment. If you are pleasing her then I'm sure she does want to please you, but is a bit insecure about it. Experiment and hopefully it'll just happen naturally, rather than you having to ask!