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.
I have tried my hand at hatching these eggs. first eggs I got were about 2 years ago. I had one hatch she a female. She followed us around like a puppy. we still have her she runs with our geese. she hates when the geese start fighting she's alwalys there to break it up. I tried again about Nov 09 bought 2 more eggs. 1 didnt do anything other qutie at day 20. tried again not too long a ago only got one eggs this time. it was a dud. I don't know if I want to try again onless I can find some closer to me. hey chickenzoo when your start laying can I buy some??