I can say that I definitely have to be in the mood to give my husband a bj. And sometimes I swallow, and sometimes he cums on my stomach or chest, depending on my mood that day.
Have you asked her about it? I know sometimes women are concerned about smell, especially after a long day. Sometimes it is more a fear of not knowing what to do. And if she has watched bj's in porn movies, it does NOT look enjoyable.... they are making those women gag to the point they look like they are going to throw up. That wouldn't make me want to do it if I hadn't done it before either.
Suggestions:
1) If her concern is smell, or residual taste after urinating from there during the day, offer to jump in the shower. That is actually how I started. Then I realized that I didn't need to have him do that every time.
2) If her concern is not knowing what to do, you can offer to look up how to do it with her. You can also do this... the next time she kisses the head, ask her very sweetly lick and kiss your shaft and make noises for her that she knows it feels good to you. IF she seems to want to do more, then again ask her sweetly to just wrap her lips around your shaft. If she does that, make a big deal about it feeling good. IF she doesn't, save this step for the next time. IF she wants to keep going, then ask her to move her mouth up and down your shaft. IF she doesn't want to do that, again, save that step for the next time.
The idea is that you are teaching her at her pace what to do and what feels good to you. Also that she doesn't have to make herself gag to do it. Hopefully she will see how much you enjoy it... And maybe even enjoy it herself. When I am in the mood, I love giving it to hubby... partially because I love knowing how much he enjoys it.