Tests work by monitoring HCG levels in your urine. If you're struggling, your levels could just be low, try testing first thing in the moring as the first wee of the day tends to be more concentrated
There are lots of reasons your period could be late, including things like stress, so if you do continue to have no sign of your AF then it might be worth booking to see your doctor anyway (they might also be able to suggest a more accure way of testing).