Senna,
You might think a lesser quality egg wouldn't develop as far along, but of my three Pekin x Rouen crosses, the two that didn't hatch were fully developed-- just didn't pip and hatch. And then there was the third that I mentioned before, that had problems hatching.
Who knows, egg quality may not have been a factor for you. I'm fairly well convinced it was for me, though. I had six Pekin eggs, six Pekin x Blue Swede, three Pekin x Cayuga, and three Pekin x Rouen in my hatch. Tossed one clear at Day 7, and the other 17 made it to lockdown. My incubator's a King Suro, and it held rock solid temps and humidity throughout. 14 out of 17 hatched clean, right on time, and are the heartiest ducklings I've ever seen, two weeks later. The three Rouen crosses having problems just seems to be too much coincidence to me.
Your mileage may vary, of course, and it sounds like you're concentrating on your Pekins from now on (beautiful ducks, I love mine!) so it's probably not an issue for you going forward anyway.