I too had been only getting a feint second line until this month when I started testing at night. Apparently the lutenising hormone doesn't synthesise in your body until the middle of the day or later. I followed the advice of someone else on the forum to test at night & it worked! Dark as dark can be, two nights in row.
Hope you get a +ve OPK soon......if not, just BD for more days than you think you need to. That way you won't be kicking yourself if you happen to O late... like me!!!