I had the same problem in my campaign... I was a complete stop at second generation and i had problem maintaining my cities because all the first generation started dying by natural causes. So i figured since i was out fighting all the time with 2G heirs they didn't have time to meet their wifes because of all the wars. So i swapped and had my 2G heirs maintaining my citites and 1G doing the fighting. After a while the 3G heirs came out.

Now i do not know it this is just coincidence or something very cleverly thought out by CA.

Cheers