Maybe the eggs didn't get incubated properly because they were buried. When you put the fakes in, the parents may have forgotten that they had a bunch of real eggs hidden under the bedding.

When I put the fake eggs in, those buried eggs weren't there, I checked and counted the real eggs then pull some out the nest.
So she must have laid all those later, and for some reason bury them. They were between one and two inch below surface.
Or if she didn't bury them, maybe the weight of the fake ones push the real ones deep in the bedding.

EDIT: Sorry for getting off-topic, I will open my own thread about this.