With most guys I've been with, I could make them cum from oral in a heartbeat if I wanted too. It wasnt why I did it, but it was nice. Even guys who previously told me they never could. Truthfully, I never gave it that much thought.
But years later there was this one guy.. eventhough it would take him ages to cum from handjobs or penetrative sex aswell, I never got him to cum from a blowjob - and as egotistical as it may be, it still haunts me to this day. It never bothered him, as he had never cum from oral - in fact, I was the first he even got close to cumming with (over and over, adding to my frustration, haha).
Believe me, I tried. Ev-e-ry-thing. Deepthroating, giving him more control and letting him fuck my throat (which seemed to get him the closest), involving his balls, different techniques and rhythms, sometimes for what seemed like hours. He got close, often a few times while I tried, but never actually got there. Like I said, I knew he usually took some time - not just with me, but always - and when we would have sex after, it would still take him some time to cum then.
He had told me he wanted to cum, and I'm possitive it wasnt as much technical as it was mental - perhaps indeed some controll thing. But having read through this thread, I sure am tempted to tell him to try to flex the next time he does get head.. It would be interesting to see if it makes a difference!