This computer is not authorized to play.....

I purchased 3 albums and 3 songs from ITunes, downloaded them to the PC that I've now had for about 18 months. I've downloaded many albums and songs successfully in the last 18 months to this same pc. But with the recent downloads I get a "This computer is not authorized to play SONG NAME . Would you like to authorize it?" I log in to authorized it. It tells me I'm already authorized. But it won't play this particular song. It plays all other songs on the purchases album. Just not this particular song. This is also happening with 2 songs on another album that I purchased at the same time. Has anyone else experienced this? Any ideas?