One of the things about masturbation is that you <b>can</b> do it too much, hard to believe but true.
If you're doing it too often, then the sensations become just kinda common place and not exciting enough to orgasm from anymore. I would suggest trying to take a break for a few days and then try again.
Another thing you could possibly try would be to get a sex toy, like a vibrator or a dildo, or a new and different one if you already have one. Maybe varying the way you masturbate would help.