Opk- how far off??

Jessica • Edd 11/9 #3 after losing 6 mo old daughter last July & having a tubal reversal in September. Hoping for a rainbow baby!
I've been doing opk since day 1 of my fertile window (which originally showed up as Friday the 13) every one is negative so far and every time I put in the negative it moves my fertile window and period day back one day.  I'm worried maybe I'm doing it wrong or missed it or maybe didn't ovulate at all :(
​Starting to get frustrated as I conceived my first two girls in one month and now this is month 3-4 (post tubal reversal) and nothing.  I'm scared something is wrong.  Any insight on these kits? I've been not drinking fluids in the am and trying to check around 10- noon using clear blue easy read.  Not sure if I should check at a later time?  Any ideas would help.  
​Guess it doesn't matter much anyway no matter what the result we pretty much bd once or twice a day :)