I think I've read online that the surge only lasts a few hours sometimes so I wouldn't worry about it being negative later on in the same day. This is why so many people never get positive OPKs I think!
They are so confusing though! For months I never got a positive OPK using the Internet cheapies, I eventually caved and got the clear blue digital ones with the smiley faces. I only use them when I start to see the line on the cheapies getting darker so as not to waste them as they are more expensive.
At least with a smiley face though you know for sure.