I dont have the strips, but I have the saliva tester. I don't think it works though because I have as much sex as possible in all the right positions and everything on days when there are ferns and still nothing.
I have tried them and they do work. Maybe you are not starting to test soon enough and you are missing the positive test, or not testing long enough and missing it. I would suggest that you try to test sooner. Doctors will tell you that you ovulate on day 14, but that is not always the case, I ovulate on day 12. Everyone is different.