purplelirpa

I'll second rangerjoe's theory. Does she like to be fingered, what about toys? If she's feeling like there's nothing in it for her, maybe you could use a vibrator on her while she gives you a blowjob? Maybe surprise her with a vibrator one day, it can be simple like a bullet for her clitoris, or even something complex like a rabbit for penetration and clitoral stimulation. If she doesn't want you to use it on her, maybe she would like to use it, herself, while she sucks on you.

Also, how long would it take you to climax from oral? Maybe she gets tired jaw muscles or something and normally initiates sex when her mouth is tired. My boyfriend can take up to 30min sometimes to get off from oral (especially if he's masturbated recently), and I'll admit that my mouth gets tired sometimes. If I don't feel like spending all of that time, what I'll often do is to ask him to jack off for a few minutes first, or even the whole time, while I suck on him. Not as satisfying, I'm sure, but it's something.

Third possibility, is she doesn't like the taste of cum. Just knowing that my guy doesn't care if I spit it out is enough for me to get over that.