Once they've tried and a baby didn't happen, drop one on the other again. Of course they will say it's too soon to try again. Close the game and try again every 15-20 minutes. Every time they say it's too soon to try again, close the game for 15-20 minutes and try again. I also recommend pausing the game when you're away for hours until your couple has at least 1 heir. Baby Boost can help a bit.