I am not quite sure what is your problem, but consider a lot of heterosexual sex.
Also enable womb view, to see how your babies are progressing. When a baby is ready to be born, press the heart on top of the cell.
And don't forget that when babies are born, they need to be transferred to a woman in lactation cell. Otherwise they'll die and disappear.
If I was doing that, I would have genes set a variable, and each kind of monster would have a different formula to calculate the actual stat value. With all formulas subject to diminishing returns, but an orc would benefit from extra hitpoint genes more than a puny goblin.