I masturbate for about 15 minutes, and nothing happens; I remain erect, but I can't ejaculate. Eventually I get tired of it and quit. I've had many "wet-dreams," but I can't masturbate to the point of an orgasm. Why is this?
I'd say you're giving up too soon, and not getting into it enough. Masturbation isn't like scratching an itch or rubbing a sore muscle – it's about building up pleasure. I recommend totally concentrating on your pleasure and your fantasies, staying with your feelings rather than dropping them, and I also recommend using lubrication. Try to get it to build up, and then keep going until you can let it go.