We need to understand what our sages said, “A son makes the father worthy, but the father does not make the son worthy.” King Ahaz, who was a complete wicked, has the next world thanks to his son, Hezekiah, who was righteous, but Menashe [Manasseh] son of Hezekiah does not have a part in the next world although his father Hezekiah was righteous (Sanhedrin 104).
I heard from Baal HaSulam that the sons come to correct what the parents did not manage to complete. Therefore, when Abraham completed only the quality of Hesed [mercy/grace], afterward Isaac was born in order to complete what was missing. Then, when Isaac completed only the quality of Gevura, Jacob was born, who is the quality of Tifferet, and completed the middle line.
So it is in all the generations—they complete what the previous generations did not manage to complete. This has continued since the sin of the tree of knowledge, for after the sin, his soul was divided into 600,000 souls, as explained in the writings of the ARI.