Checking Egg Fertility After Failed Incubation

[Inga73]

Can you crack open eggs that have been incubated for three weeks and still use the bullseye to see if they are fertilized?
Someone I gave hatching eggs to said she had none hatch, and she had a friend open them and see that over half were not fertilized. This was a surprise for me as I have been hatching my eggs at great rates, and we have good rooster to hen ratios. I can tell by the hens feathers that they are getting lots of loving, not to mention I see it happen often!
I feel like it's more likely an issue with her incubator, as she mentioned she had zero hatch from another batch right before she got eggs from me.

Thanks for any insight!

 

[nuthatched]

After that long of incubation, the eggs would have rotted or exploded. There will be nothing to see. It's likely her incubator, have her calibrate it.