You shine a small bright light into the egg. Like a pen flashlight. If the room is dark, the inside of the egg will glow. A fertile egg will show a roadmap of red veins over the inside of the egg. And infertile one will be yellowish or clear inside.

I would guess it might be the moisture in your substrate, but I don't have a lot of experience with eggs yet. I do mine substrateless.

Gale