Well.... there could be so many things causing the issue, and it will almost always be a combination.
I've thought of more. My 108 egg, home built is now hatching. Currently 72 out, and two days left to day 23. (Unless very obvious activity, the 24th day is off and clean it up day for me.)...
Possibly:
A. Besides fertility there are many other factors involved when incubating ones own eggs. I have experienced this very problem back around 1984 or so... and found (in that case anyway) it was the egg producers themselves. After two fails, switched to stolen eggs from a good friends...