Yeah, it sucks when you get a "scraper".Ooh, this is a good question. I had a boyfriend once who was quite hesitant to let me try, because he said that nobody had ever been successful at it before, and that he always felt like it was some kind of a let-down when he couldn't orgasm. He felt uncomfortable telling girls what they were doing wrong, because they tended to take it personally or to just give up in frustration. He felt pressured to respond a certain way, whether he felt like it or not. I could relate quite well, because I have never yet orgasmed from oral sex, mostly for the same reasons.
Eventually, he let me try experimentally, with a relaxed attitude about it so there wasn't any pressure to orgasm, and with an open mind about the giving of advice. Sometimes the problem is psychological, and can be overcome. He discovered that he actually liked oral sex when all of the negative or stressful associations were removed. Once he was relaxed, he orgasmed quite easily.
I think a lot of girls assume that guys are all into oral sex, or that there wouldn't be any reason for a guy not to be. Likewise, it seems like most guys feel inclined to brag about enjoying giving oral sex, perhaps as an enticement when trying to lure a mate. Even by telling you what I just have, it is possible that I will be seen as bragging. I'm not trying to say, "I'm so good at oral sex that I gave a guy his first oral orgasm," because that isn't the point at all. I am trying to say that sometimes what we expect about "guys in general" or "girls in general" isn't true, and there are many emotional/psychological issues that have to be considered.