Have you tried the baby boost? How old are the parents? after 35 years most of them will require a babyboost to have a baby. also make sure they have showered, been fed, and are healthy. also from my 'notes' on my games i have noticed that they are more likely to have a baby if they are not tired/exhausted. Hope this helps!
(Edit) You can make them try for a baby every 15 minutes. Unless they say No its too soon!