I tunes not loading current library and saying unavilable on PC

I have an older Windows based PC that has my i-tunes library in it's entirety. My i-phone and i-pad both reflect what is on that PC and I have i-cloud music turned on. When I try to download the i-tunes program on a newer PC it downloads and shows most of my music either not available or not downloaded? I have removed and reinstalled 10 times now with the same results. At some point I am going to need to back my devices up on something other than the old computer and it is beyond frustrating at this point.

Does anyone have any ideas as to why this constantly happens?