Stimulate yourself in a more feminine manner... increase the build-up time, get more of your body into it, and find a way to use lots of mildly-pleasurable repetitive motions to get yourself closer.
Doing it the male way is what's causing the weak orgasms. And yes, I've experienced this too. When doing it the male way now, there's basically nothing. I wonder if it even counts as an orgasm. But when I get in the tub, spend some time really getting into it, letting my mind go, just enjoying it, and working at it for like half an hour before finally going in for the big O, it is freaking AMAZING!!!