Disconnect between iPad and iPhone/iMac

[theatergeeknyc]

This is probably a stupid question, so I apologize in advance, but it used to be that my iPad, iPhone, and iMac all had the same music--when I downloaded an album on one device, it would automatically appear on the other two, as well. However, in the last few weeks, I've noticed that my iPhone and iMac remain in sync, but now the music I download on my iPhone and iMac does not appear on my iPad. Do I just need to download it again, or is it possible for me to get them to automatically download again?