Is she paying attention to the egg? If not, you may want to take it out until the next egg is laid and then put them in there for her to sit. She may lay as many as 5 eggs; she may stop at the second one. I understand that when it is the first time, it may be only one or two eggs.
I say this so that she does not break the egg. Do you have something in ther nest, such as a rag or something, to give a soft place? It could protect the egg, too.