I Recently Missed My Period (4dpd) , I Am So

I recently missed my period (4dpd) , I am so timely on my period I could have dated the month and hour even by how timely it was but now nothing?! I have taken 3 pregnancy tests but still confused ..... the first one had a faint line and the second had a faint but more prevalent line and then the third there was no positive line at all? Could I not have enough hcg level to register? Should I and when to retest?