According to here:
The Aruch HaShulchan was originally written and published in sets of approximately 20 simanim a piece. When its author died, he left the remaining packets to his daughter to finish publishing. Apparently, she never finished publishing some of the last booklets. No one has seen the manuscripts since.
The Aruch HaShulchan left all his manuscripts to his daughter, but then the war broke out, and they were sent to Israel. Included in his manuscripts were the Aruch HaShulchan He'asid, which was printed by Mossad Harav Kook; D'rashos kol ben Levi, which was printed; the second chelek of Or La'yesharim, which was not yet printed; and the rest of the Aruch HaShulchan, of which only Hilchos Nedarim was printed.