Frank - I've had the same problem. The last few girls I've banged, I couldn't come the first time we did it. I think it had to do with being anxious. Then the next time I was able to get a nut.
What's funny is that when I'm pretty comfortable with a chick, I like to go for multiples. Like at least three times, right? Pumping for probably more than an hour total.
Scars - I remember being like that when I was in my late teens. I could nut 3-4-5 times without stopping. As I got older, I had to stop for a few minutes after each nut, but I developed a lot more staying power. I can still pound it out like a boss now, but it generally takes more mental effort to make myself relax to get that first nut.
Also, I've heard this has something to do with the pelvic floor muscles. If you're really tense, they tighten up so much that you can't even nut. Last time this happened to me, I had a horrible attack of prostatitis and was in a shitload of pain for about two months.