I flush every two weeks on the advice of my naturopth doctor. I took huge amounts of Ibuprofen and other OTC and prescription pain killers that resulted in what the doc. called an unusaully blocked liver. I must confess a lifetime of a bad diet also helped contribute to the problem. I don't remember ever doing a flush with no results, but there have been a couple where I only passed a few stones.
My advice is to keep flushing until there are no stones (or only a very few) two flushes in a row.
Fasting on apple juice for 2 or 3 days before a flush usually helps produce good results and clean BMs where the stones are easily seen and counted.