Two songs will purchased will not play on my computer or ipod

I downloaded around 17 songs during the last few months and two songs will not play on my computer or ipod. When I try to play them I get a message that my computer is not authorized to play this song and would I like to authorize it. I go through the steps to authorize the song, but when I try to play it, I get the same message again. I have around 500 songs and I can access them all except for two songs that I downloaded recently. I talked to a few apple support people on the phone and no one could help me figure out what is wrong. I am nervous about purchasing more music. I don't know if this will be a hit and miss, where some songs play and others do not. I never had this problem for the nearly ten years that I purchased songs. I hope someone knows how to fix this.