i am trying to re download music onto my ipod nano 6th generation but its not letting me-grayed downloaded button help please?

I'm trying to download my previously purchased music onto my ipod nano 6th generation but the button to download it is grayed out and tells me its already downloaded....earlier i was trying to download them to a different device....is there any way i can redownload my music or at least a way to save it for purchases later??