Hey I'm sorry for my bad English but I can't set the fertility chance with my sims, and that's why they don't get pregnant. I've really tried everything but it just doesn't work... but everything else about the mod works.
Hey!
Are you sure that your Sims are in the fertile days of their menstrual when they're trying for a baby? If not, you have to wait for the right time for them to have the highest chance at conception.
Also, make sure that they're not using birth control when WooHoo-ing, or just play around with settings!