Sometimes the urine will evaporate from the window and it will show a false negative!
It could possibly be a chemical pregnancy whwre the hpt will first show positive then negative then you will get your regular period.
Did you use first morning urine for each test? It's best to test first thing in the morning with home pregnancy tests. Also, how long has it been since you had sex or missed your period? That would have an effect as well. You may have tested too soon.