I just hatched a batch two weeks ago that I almost tossed. This was my first batch this year, and I had a lot of problems with the incubator temperature. Once It got to 107, another time 104, and several times it was in the low 90s. We had had several very cold days (-10) and that didn't help!
I had almost given up on them, and was going to toss them, as none of them looked like they were even close.
I broke 3 open to see. One egg wasn't fertile. Another had a chick in it, but it had died a while before. The third one had a little chick in it, still alive. I felt horrible, but at least then I knew.
So I gave them another week past the hatch day. All total I got 4 out of 41 eggs to hatch. Not bad considering all the problems I had.