How old are the eggs?
Seperate them if you have not done so already, to protect the other egg from mold.
Mold can kill an egg, usually due to, too much moisture, but not always. Colapsing egg and mold within the first week is a good sign of a infertile egg. However the best sign is it becomes smelly.
Candle the eggs with a small flash light. If the egg is pink and shows veins then it's fertile, if it's yellow, ususally it's not fertile. However don't through out an egg unless it smells and/or way past due date.
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6.16.1 Leos
1.0 Peach Fronted Conure
0.0.2 Red Ear Sliders
1.0 Pembroke Corgi
0.2 Cats
8 fish