Just wait and keep trying every once in a while. This happened to my couple when their daughter turned 2. After a while, I managed to make them try for a baby again, but they weren't sucessful. Later, they did try a second time and they now have another daughter.
Also, like other people have said, I think it depends of how much they want kids. "Maybe"/"Definitely" couples do seem to want to try for babies sooner than the ones in which one of them is "Not Really".