Just keep trying. I think you can have one"Trying to make a baby" every 15 minutes or so. Any other result of dropping one parent on the other, you can keep dropping the parents on each other until they try.

There is a baby boost shot in the medicine section of the store you can use if you really want more children, but as long as you have one child, you have the next generation for the house.

I've found that buying the perfumes/colones seem to make them more likely to want to try, drop them together when the perfumed one is "celebrating the marriage."

How old is your mother? I think if they get over 40 that they either can't have children or the chance of getting a child drops a lot, even when using the baby boost shot.