Can't orgasm during oral sex

I don't know if anyone else has this problem, but I can't orgasm during oral sex. All of my partners have been very enthusiastic, but only one has been successful in satisfying me during oral sex. I'm not afraid to communicate what works and doesn't work for me, but no matter how clear I am with communicating, nothing works. I end up just being there while my partners eat for their own satisfaction. I don't know what to do to help them help me, but it's frustrating. Any suggestions?