On my blood test results the normal range for the LH surge is anywhere from 9-74 IU/L, so if the test only picks up 20 IU/L's and you have a lower range surge, then it might not give you a positive reading.
My sister got pregnant when she BD'd the day of her "negative" test. She had been getting no lines all cycle, and just a faint line on the day of her surge. Her doctor told her to take that as a positive.
I know that how much I drink and how long I hold my pee really makes a difference to the test as well.