Join the club. Been with this girl 4 years and haven't been able to cum from oral at all. Not for lack of effort and I feel guilty sometimes about how long she is down there. My ex was able to a time or two. The real winner was a fuck buddy. She was crazy (seriously) and would beg me to fuck her throat. One time she blew me in an adult theater and I came in her mouth. It was a complete surprise and fucking awesome. Maybe you have to up it a notch on the excitement scale? I'm just guessing on my own personal experience.

Best of luck!