I've downloaded songs from my computer to my ipad and some of them still need downloading but it wont download

I downloaded songs from my computer (that I got from my iPhone) to my iPad and some of them have a cloud symbol next to them and I tap it and it says Download Over Mobile? I haven't got Wi-fi or a sim card in it but that doesn't matter because I've already downloaded 1 album and it works. How come these songs won't play?!