Ovulation Detector Kit

I have been using Ovulation Detector Kit for two months now just trying to monitor when my ovulation period comes. We bought the brand called "Answer" which can be bought at Wal-Mart it's the cheapest brand. We bought the 7 piece kit but I already consumed them and so hubby and I decided to get some more and this time we got the 20 piece kit in strips. I started last month but we missed it and for some reason it didn't show the dark line on the test window. Me and hubby didn't understand why but maybe we started the test later than it was supposed to be. This month I tried again and so far we are seeing the dark lines on the test window, hopefully things will work out good and we would succeed in conceiving this time. Wish me luck friends!