This is what I've wondered myself. I started using them this cycle (5th cycle) and they did get darker each time I did them.....I stopped once the 2nd line got almost as dark as the control line and my temp had showed the drop on FF so it coincided. I believe some ppl never get really dark 2nd lines other people do.....? thats what I've heard.

Good luck! Using OPK's is such a PITA I reckon!!!! I forget to use them.