why is it not going lighter?

I've been taking ovulation test since June 11 th. I ovulated on June 15 but my question is why aren't my ovulation test going lighter? They always go lighter after time and I feel like it went lighter then staying the same. Why would this be happening? It looks like it's getting kinda darker but some say it's staying the same. Is it getting lighter,darker or staying the same. By now I thought it would of been lighter like June 11 one.