Goose breeding times

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We have (what we believe to be) 2 female White Chinese and 2 male White Chinese geese. We are getting on average 5 eggs/week which is why we believe 2 female. We started incubating the eggs when we were seeing geese mate. We have 5 healthy goslings from 2 incubations at the moment. But, we've have had 2 incubation periods now where we had no goose eggs hatch.
Does anyone know if they only breed at a certain time of the year? We don't' want to waste anymore eggs if there is not a chance they will be fertile and productive.

Thanks,
David