Cat won't get pregnant

I've had my cat try for baby with a another cat at least 20 times and they still won't get pregnant. I tried a mod that has a 100% for them to get pregnant but that doesn't work either. My household isn't full either. Also, both aren't spayed or neutered so that isn't the problem. I'm really getting frustrated. I do have mods but nothing that would interfere with pet pregnancy. Please help ☹️