If you were going to use OPKs, I'd suggesting starting from about CD10... depending on how long your cycles are.

OPKs don't work for everyone firstly because the LH surge can come and go within a 12 hour period... if you test every 24hrs, you can miss it easily.

Or, like Tanya said, your LH might normally be high, and will register a surge when it isn't actually a surge... Also, sometimes your LH surge isn't strong enough to register, the OPKs can look for a higher conccentration than what is normal for your surge, so you don't actually get the line that is equal or darker to the control.

We had scans to monitor O, and temping and CM and cervix position were in line with what the scan showed, and OPKs weren't always helpful. I would try temping and checking CM and cervix position also :-)