Nehemiah 12:1-7 names 22 chief priests and Levites who returned from exile with Zerubbabel the son of Shealtiel, and Jeshua: Seraiah, Jeremiah, Ezra, Amariah, Malluch, Hattush, Shechaniah, Rehum, Meremoth, Iddo, Ginnetho, Abijah, Miamin, Maadiah, Bilgah, Shemaiah, Joiarib, Jedaiah, Sallu, Amok, Hilkiah, Jedaiah. Nehemiah 12:12-21 names a descendant of each of these priests except for Hattush and Miamin. These were not generic descendants but rather those who became priests (Nehemiah 12:12). Thus the reason Hattush's descendant is not named may be because none of his surviving descendants became a priest. As for Miamin (Miniamin in Nehemiah 12:17), he appears in the later list but his name is not followed up by any descendant - or so it seems - so a copyist error is often assumed here.
However, the syntax supports an interpretation that Piltai was a descendant of Moadiah who was also a descendant of Miniamin. It may be either that Miniamin and Moadish were son and father or that the family trees of the two priestly families converged so as to share the same priestly descendant, Piltai. For example (referring to the current British Royal Family), "of Prince Charles, of Queen Elizabeth II, Prince William" does not mean the descendant of Prince Charles is omitted but rather Prince Charles is himself a descendant of Queen Elizabeth II whose later descendant is Prince William.
The theory that Miniamin and Moadish were son and father is supported by the curious distinction of familial terms in Nehemiah 12:7 and 12:12. The priests listed in Nehemiah 12:1-7 are described as "the chief of the priests and of their brethren in the days of Jeshua" (Nehemiah 12:7) whereas the priests listed in Nehemiah 12:12-21 are described as "priests, the chief of the fathers" (Nehemiah 12:12). Miniamin and Moadish cannot both be chief of the "fathers" (i.e. the head descendant of a family line) if one is a son of the surviving other. However, they can both be chief of "their brethren" (i.e. the head descendant of each generation). Since each of all the priests listed in Nehemiah 12:12-21 is indeed the only descendant of each family line, including Piltai descending from both Miniamin and Moadish, it can properly be said that they are the chief of the "fathers".