Opks aren't getting darker...

I don't really understand opks all that much because I just started using them. Shouldn't they get darker as days go on though? They all look the same to me they never get darker it seems like. The 1st pic is from 2 days ago and one is from just now. Anyone want to fill me in on how these work? All I know is that when the 2nd line is as dark as the control line then it is positive.