Hey. Since you don't know about altering Sims fertility, you must be playing on the Menstrual Cycle mode. People with a menstrual cycle aren't fertile at all times, they are only fertile during their fertility windows. So you either wait for that time during the cycle or alter the settings to the Simple Pregnancy Mode where you change the pregnancy chances on your own.
Read more about it here: https://wonderfulwhims.com/menstrual-cycle/