low fertility king

[Tricky Lady]

Well I guess that's just a case of bad luck.
IIRC you could "raise your king's fertility" by fighting a few battles, but there might be some mist in my head as it's been some time since I played MTW (now that I come to think of it: I should start a MTW campaign again).
It happened to me only once that I lost a game when my king died without heirs.

BTW, welcome to the Org!

EDIT: There is a cheat code to make sure a son is born in the next year, but -aargh!- I can't remember it! You gotta search the forums, or wait for another member to reveal it, sorry.