If they are any older than 2 weeks I would say infertile sorry

After about a week of incubation you should start to see a change / blood lines through the egg.
If one has hatched already from that same clutch, I would guess those were never able to hatch in the first place.
First time breeding with that pair? Or are they young birds? Would not be worried if that was the case.
If they are well proven breeders it could just have been one of the pair being a little ill at the time.
The fact you got one is a good sign
