You need to keep flushing until you are clear of stones 3 times in a row. Those 2 where you got nothing out could have had other circumstances that caused you to get no results which could be caused by different things, who knows. I would give it another shot. Just make sure to keep it up every 2 to 3 weeks. Too soon and stones do not have time to move forward, too long and you are sort of starting over each time. Hope this helps.