Hi Lucy the sticks detect your luteal surge which is what triggers ovulation, ovulation itself normally happens 12-48hours after the surge but the average is 36 hours. When are you testing? The luteal surge normally happens during the morning so if you are testing with FMU you may be detecting yesterday's surge or miss it all together. It is best to test late morning/early afternoon for accuracy but make sure you restrict your fluids and save your wee for up to 4 hours before testing.
HTH!