my music is mostly grayed-out in iTunes

I have this issue where every day when I go to play music on my iMac at work many of the songs in my library are greyed out and won't play. They are mostly songs that have been in my library for a long time or are in playlists. I suspect they are mostly songs from my iTunes Match service that do not exist on this computer. When I reauthorize this computer in iTunes after a while they all come back. The question is, why do I have to keep doing this everyday? I don't have this problem on my other computers, and when I authorize this machine is says I have only authorized 4 of 5.