Great question. Which flush have you done? The flush I use has Orthophosphoric acid in it. Ultraphos. That seems to soften the stones and it also allows the Common bile duct to expand so there's no trouble passing whatever is in the gallbladder or liver.
I've passed stones as big as what is now in your gallbladder. A few even bigger. They're in my freezer here at the office. I show them to folks every now and then just to give them a reality on it. I saved a few of the bugs too. When I show folks, that can't believe it. But I'm still here so they can't deny it.
Anyway, try a different flush. If you want my recipe email me and I'll send it to you.