I used them only in my first month of trying and found I got two light test lines on the first 2 days of using them. I was so excited and kept testing waiting for them to get darker and ... on the 3rd day of using them, the lines disappeared altogether. I tested every day from then on, once mid morning and once early evening and got nothing. No more hints of tints or anything from them, so I've sacked them.
Someone else had the same experience and posted here saying that she contacted the manufacturer, who told her that in the case of 5% of women, they do not work at all. They don't publish this fact, so it's worth considering whether you are in the 5% or not. The only way to tell is to keep trying them for maybe 3 months.
Best of luck as I know these things are really frustrating.