Just found this info, it may help....

I am so confused... LOL
Here is an example of my OPK's for one cycle:
On Day 14 of my cycle, I tested at 11:30 p.m. The test was negative (the result line is fairly dark, but not as dark as the test line.)
On Day 15, I tested at 10:00 a.m., only 10.5 hours after my previous test. By this time, the test was positive, and my LH surge had been detected.
Again that same day, I tested at 4:30 p.m, only 17 hours after the previous day's negative test. This test was positive; I was still surging.
And once again that day, I tested, this time at 8:00 p.m. The test was now negative, indicating my LH surge had ended-- and this was still only 20.5 hours after the previous day's test. Therefore, my entire surge lasted less than 24 hours. If I had not taken the additional tests, but followed the instruction to test "every day around the same time," I would have taken a test later that night and missed my surge altogether.
So, if you are unable to detect your LH surge, try testing twice a day! It is possible that you are not ovulating, of course, but this is not a conclusion you should draw based only on a lack of positive OPK's. If you still have concerns, talk to your doctor.