Why doesn't my library match on ipad & windows 7 when both are clean downloads to each device from the cloud?

As I have done this, there are several pieces of music which are missing on one of the devices. When I tried to find a particular one that was missing, I tried to just buy it again so that my music would be the same across devices. When I tried to purchase that item (again), a window pops up telling me that "I have already purchased this item and would I like to download it." Why? Why would that item not have shown up with the other two hundred pieces so that I would not have to find it?

Next. Another item which shows up on my iPad Air (in the Cloud), is not available in the *store* (or the Cloud of course) to play on my Win7 computer. Both devices were clean creations so how can this be? How can I get a particular piece of music in the Cloud to show on one device, but not another? None of my music is on the device. All are in the Cloud and show the cloud icon.

All of my music has been purchased through the years from the iTunes Store.