No there is no secret. For some reason all flushes are not the same.
After flush 20 I started the Oxypowder flush, on their website under Superfos30, that seems to work for me. I have done another 7 or so now.
I never get big stones, may be I don't have any. Since I had one 28 mm stone, I have been using chanca piedra and other softeners, so I kind of expect to get smaller pieces.
Sometimes I create my flush from several different flushes.
I am still looking forward to Lorelei's answer.