I may be wrong but aren't you supposed to listen to the sound that the tapping creates, an infertile egg at 30+ days will sound hollow. You can also try the heat method, take them out of the incubator for a minute and let them cool, the bigger end with the air cell should feel cooler than the smaller end, this means theres a chick inside retaining the heat. And theres also the leaning method which drafthorse told me about, at 2+ weeks if the egg leans to the smaller end on a flat level surface you have a chick growing. Of course dont throw them out until you know for a fact they're duds, otherwise you could end up throwing out a good egg.
Good luck.