OPK too late??

I ordered my opk's on amazon...but may have for them too late. I had EWCM yesterday, but didn't have my tests until today... Today test showed an empty circle (no smiley face indicating ovulation). All other signs bbt and cervix position point towards ovulation. Could I have missed my LH surge yesterday?? How many days do you get a positive opk for? I have always stopped testing after getting a positive.