Letrozole, High Opks, No Peak?

Today is day 16 of my cycle. I took 5mg of Letrozole on days 5-9. I experienced a longer than usual period (8 days as opposed to my usual 5 or 6) and I haven't ovulated yet according to ClearBlue Advanced Digital. I got low readings until CD12 where I got my first high. No peak yet. Without letrozole, my peak reading usually comes at around CD14 or CD15. Would letrozole cause me to ovulate later? I'm testing twice a day now. Early in the morning and then again at 6 or 7pm to see if I can catch the surge.