Ovulation Confusion

So I try to take an ovulation test twice a day. One when I get for work and one in the early evening when I get home. The last few I have taken the line has been there but not dark like it's suppose to be. Well long story short today was suppose to be my highest day of ovulation(33%) and the line has been more faint than the last couple of days. Has this happened to anyone else? Is this normal? These are from this evening