Brooke,
The other things I have found for those who have difficulty getting a positive OPK is that;
1. It can depend on the quality of the OPK you use. I never got a + with the ebay bulk type (cheap ones) but I did with the lullaby brand, and,
2. You can easliy miss the surge if you only test once a day if you happen to be a person whose surge is only very short lived. For example, if you test everyday at 5pm and the surge occurs earlier in the day (ie 11am) you may only get a faint 2nd line by 5pm when you test. I have had to use HEAPS of tests in order to pinpoint a true positive. I have used 3 tests already today, all showing a different "strength" on the test line. Costs a fortune but I find it's the only way I can get a positive result.

HTH and clears up some reasons why some women find it hard to get a positve OPK.