Darkness of lines

Elaina`sMama • Got BFP on 11/18/15 with baby #1 - she was born on my birthday, July 25, 2016. Planning to start trying for #2 in July 2018 😊😊😊
When should the lines start being darker? I got my BFP on Sunday (vf) and have tested like 20 times (all positive and 4 digitals that say 'pregnant') but even today (the day AF was due) the lines are not darker???