Check the fertility odds via the toilet
With enough money they can get fertility treatment which boosts it to 100% located in the wonderful store option
Sometimes you have to ask the partner to stop using birth control (both partners) in the wheel menu, I believe it's next to last
Check in settings for the odds if you're doing it by cycle (0 to 100%) 80% is typically what it's set to
All else fails toggle the mod back to the game settings of "try for baby" and "woohoo"
It takes a little getting used to