If they are in their 50's they can still get the "adopt a kid on the doorstep" event. I've had that happen twice in different families.
If they are younger, you can try adding these things:
Baby Booster Shot
Male colone (from the varied section of the store, squarish yellow bottle)
Female perfume (from the varied section of the store, blue fancy bottle)

The colone/perfume seems to make them more willing to try, and the baby boost is suppose to make them more fertile (but the effect lessens the more times you use it, so choose carefully with it)

In two of my games I've been trying to get my peeps to make babies for some time. One family had three kids already and the other four, but I like to see a full house. I had doctor's consultations after the couples failed three times in a row, but there was no recommendation of Baby Boost.

I just tried again and the couple with four kids got twins. I then switched to the other slot, and they had twins, too! I'd only once had twins before, in my very first game.