Opk question????

Ive tested throughout my cycle (now on cd18) only thing that has come up is a very faint line when I test in the morning but then disappears when I test late afternoon?? Apart from this it's completely clear on from cd9 - cd18?? I usually have 28 day cycles??
​Any thoughts ladies?? Do you think I'm just going to ovulate really late or might have missed it? Only started testing am and pm last few days!