If your store is still stocked heavily on wave 1, it might be some time. Of course, wave 1 came out in late March and is still shipping so it may just be patience in getting them.
He-Man/Superman is selling quite well but Skeletor/Lex Luthor is not. I would guess that, Lex having been available so many times before and with Skeletor missing the staff, people are less interested. He-Man and Superman are 2 popular characters so it was a good combo.