Missing Songs

I have found that more than once my old purchases have disappeared from my account and my iphone. I purchased George Strait's greatest hits and it's nowwhere to be found. I had about 25 songs and now zero. I have checked previous purchases. No help at all. I also purchased three of Kings of Leon's albumns and I only have one now... This is the fourth time this has happened. What I'm worried about is, if I didn't try and listen to these songs, how do you know when these songs get deleted?