Separately purchased song from an album isn't downloading from iTunes

In the iTunes app on the Mac, and iOS, a particular song doesn't download from one album but the rest of the album downloaded. It has been stuck like this for several hours and I have tried to restart the download for the entire album but the same problem pops up again. But, when I first purchased only the song from the album I didn't have the issue. When I bought the entire album, that's when the issue popped up. I'm dealing with the same problem for many other albums too. The songs which I've purchased separately, (from the same album) do not download when I buy the entire album.