Finches Not Mating?

I have 1 male and 1 female finch. They are about 1 year old. They laid an unfertilized egg on 17 aug 2015, just after that they stopped mating. In the mornings my male bird arranges the nest, but in evening neither of them go in the nest nor do they sleep together. 1 month ago they used to sleep together and used to mate, but now days they aren't. In the morning hours they play together, they never even fought. Can anyone say what's the problem and give me the solution? Thank you.