CREATE TABLE table_2629 ("District" TEXT, "Vacator" TEXT, "Reason for change" TEXT, "Successor" TEXT, "Date successor seated" TEXT)

Name the date successor seated for failure to elect
SELECT "Date successor seated" FROM table_2629 WHERE "Reason for change" = 'failure to elect'
SELECT "Date successor seated" FROM "table_2629" WHERE "Reason for change" = 'failure to elect'