If one did not complete his Divine mission on Earth the first time--if he did not utilize and perfect his soul through Torah and mitzvot according to his capacity--his soul must descend a second or third time, etc., to complete the unfinished work. At the time of the Resurrection all bodies will return to life. The portion of the soul utilized and developed during the first life will enliven the first body, the portion of the soul developed during the second life span will enliven the second body, etc.(Rabbi I. Altein in The Yiddishe Heim)

Reincarnation and Resurrection