This is almost always a license issue. Ensure you have the correct .RAP files installed. Without these, the game sees the data but cannot validate the "purchase," so it hides the songs.
One of the most unique aspects of Warriors of Rock was its ability to import songs from almost every previous title in the series. While these imports originally required a small fee, they are now preserved in various community "all-in-one" packs: guitar hero warriors of rock ps3 dlc pkg