Unfortunately, it seems, it takes a few flushes before any stones come out for some people, including me.
My first flush prodcued about 50 very tiny stones. I was really disappointed.
The 2nd only about a dozen small stones.
The third was my most successful with about a dozen stones again but all 3cm + in size!
4th, a complete failure - nothing.
I'm hoping the 5th, which I'm doing tomorrow, will be huge!
All I can say is to hang in there! It might take you a few flushes to get there and if it helps, they get easier every time!
Good luck :)