Grizzer, I don't think it is because you left it longer than 10 minutes. It is possible the surge is yet to come, the body sometimes gives it more than one go, or, that you didn't quite catch the surge sometime between when you took the opk on Monday, and today's.
I am no expert. But there were times when I did opk's in the morning which were positive and I know we are not supposed to do them in the morning. But if I hadn't I most likely would have missed the surge by testing in the afternoon/evening. I confirmed it was the surge through charting. That's why I continue to chart - it really helps me know when I have ovulated, combined with using opk's.
Do you have any other signs of ovulation? I get the O pain most months, which also helps me know when it happened.
BTW, I always do the opk's, stick them in my underwear drawer and check on them in 5-10 minutes - like you I keep a little collection until I know I've ovulated. I am glad I am not the only one who does it 
No spotting for me yet. Now I am doing the "irrational talk" to myself: 'Maybe I won't get AF. Usually I have spotted by now. Maybe I am pregnant but it is a late implanter". "Boobs not as sore, but it is the morning, and they always hurt more in the evenings", "As for the low temperature, it isn't really the death Nell. I mean, it is still above coverline"...
OMG! I have TTC madness! 
You would think, wouldn't you, after 5 years of ttc, I would have a modicom of good sense to know when it is doomed. Hope springs eternal! Which is probably just as well.
Thank goodness I have this thread to unburden myself on - trouble is, I am afraid I might be burdening my online friends instead 
Ignore me! 