That can and does happen to anyone, even during actual intercourse. It could be completely mental, such as not having the correct stimulus (try switching magazines :P), but it could also be caused by lubircant. Some, maybe most even, contain a retardent (lol) called 'benzocaine', which basically numbs your weiner so that you can last longer. Great if you're trying to impress some girl, stupid if your just whacking off.
Try a different stimulation. Maybe switch hands or use both. I like to "edge" I find it very exciting to bring myself to the edge then stop for awhile then start again do this a bunch of times then let go. This bring on a very different feeling. Do you use porn when you masturbate? Maybe look at a different type of porn, change things up a bit see what comes up