Mel,
OPK"s can be difficult. I had real difficulty when I first began using them.
Some women get a second line just prior to the LH surge which is darker than any line before. When this happens you should test again 6 hours later. Very often you will get a positive or almost positive. YOu will then see the line fade to nothing again (or a very feint second line).

A very dark almost as dark as the control I would take as a positive and get to work!

The LH surge is detectable for some women for 24hours. I am lucky to catch a really truly positive one. ( I do on clomid but that is to be expected as by rights the LH surge should be stronger on clomid). I can test at 2pm and it's almost positive and then by 8pm it's feinter again. I think there are many women with experiences with opk's like mine.

This doesn't mean there is anything wrong with your body it's just how detectable the surge is in your urine. I drink a lot of water and herbal teas so this would contribute due to a less concentrated sample.
It would also be a factor when the surge happens. For example if the surge began at say 11pm yesterday by 2pm today the line on the stick wouldn't be as positive as it may be at 10am today... Do you follow or have I lost you???

Anyway, OPK's are extremely efficient when you get them right. Getting them right may mean testing at intervals throughout the day around the time you start to see a darkening line. This would only be for a couple of days each month...
I would urge you to not give up on them as they are a handy little tool once we get them right! I hope that helps...
Many obs suggest you test in the morning - however the manufacturers say between 10am and 10pm is best... With the optimal time being 2pm.