OPK miss an LH surge?

I have not yet had a positive OPK but I am showing other signs of fertility. Cervix feels soft and open and experienced lower abdominal pain for about a half hour earlier today. I have been testing w/OPK once a day for a week but not at the same time every day. My last period was June 20 and I typically have a 28-31 day cycle. So is it possible that the OPK didn't detect an LH surge and I am ovulating now?