Talk to the Pokemon in the yard and if it says they are brimming with energy, they can't be bred together.
If they have the same defense DV and special the same or off by exactly 8 they can't breed together since those are the DVs passed down during breeding and the game thinks they are related, which causes the game to stop the incest.
This can even be caused if the Pokemon randomly have similar DVs and why you can not breed two shinies together since they are guaranteed to have those DVs the same.