Jenny - with the OPKs it is still pretty easy to miss the +ve. It's best to test more than once a day if you can. The ones I have recommend testing at around 11am or between 6pm and 10pm. If you can I'd do both times. The surge doesn't last too long so I think if you test only once a day then it is easily missed.
Personally I didn't fancy the idea of testing at work, so only did it at night. And yep, I missed it.
Are you using your saliva test too? How's that going?
Good luck!
looking at my list looks like Mammawannabe is next on the 11/06 . . . . Fingers crossed for you hun x