If a faction is stagnant and expands very little there are fewer heirs. I have occasionally seen a faction of at least eight territories expire for the lack of an heir. Although my assassins were a factor, this faction, the Brutti was unusually timid. Even large factions will suffer a lack of heirs if there is little expansion. My 48 province RTR Unified Rome has expanded little for some years, hence few heirs, or rather very few. King Azzole said so already, but it needs underlining. Expansion is key to having a good supply of heirs. There is no bug. CA probably engineered this feature so that players do not forget they are playing Rome Total War not Civilization III.