But I paid for those songs!

I’m running across a couple of songs that used to be downloaded (I’m pretty sure, anyway) onto my iPad, but now they show as NOT downloaded. When I try to download them, I keep getting a pop-up saying “Purchase of this item is not currently available. This item is being modified. Please try again later.” How much later? It’s been a few hours now. And when I go through the iTunes “Purchased” link, only one song shows up as not downloaded (with the same message popping up.).