Negative opks but EWCM anyone ?

Laiz • Baby#2 Due in May,2015
I been doing opks since CD6 which is May 10  till today and all are negative, but since yesterday I been having ewcm  and my predicted O was for today. my CM goes with my expected O but no opks have come back possitive. I have read that to some women their LH is too low to be catched by  the opks ? Anyone has had a similar problems ? I am ttc baby #2