This isn’t uncommon. I put it down to your body just getting used to edges and ruins if you’re having those too. It’s nothing to worry about.
Just push through, give yourself a few ‘fizzles’ and they’ll start to feel better and better, and normally after a few they’ll be feeling good (multiples one after the other tends to work). And then the next time you have a full session all the way to cumming it’ll be back to normal.
It’s a strange thing, sometimes it doesn’t happen at all and after some denial you have the best orgasms of your life. Those it happens for tend to find there is a duration that works for them where it’s great, but then tips over into ‘fizzles first’.
But for many, it doesn’t happen at all. I guess it’s just your body enjoying messing with you.
The mean way I use it for those who can experience this is letting them cum, and they get a ‘fizzle’ and it’s not at all satisfying. And then inform them that was it, all they did to beg and convince me they could cum, that it’s clear their body didn’t want it. So now they’re denied again.
Not that I’m planting any ideas, you know.