Anyone used the Clear blue ovulation predictor

I have used it for one month an it was saying I ovulated. This month it always came up as low fertility but when I looked at the lines on the day I was supposed to ovulate it had the two lines and still said low fertility. I took two more tests each day an it showed low fertility, but when I looked at the lines it only had one line. Does this mean I might have ovulated earlier then expected? I did see big drop an rise in my bbt during those days.