Ovulation Test Confusion

I've been having a little bit of trouble understanding the ovulation test. I know for it to be a full positive the second line has to be the same/darker than the control line. But for the past week I've been taking two ovulation tests a day and both have been getting darker everyday but not a full positive. I was supposed to be ovulating today but today the line doesn't seem to be there? Does that mean that is has already happened or shall I carry on testing twice daily until I get a positive?