Not all previous purchases available for upgrading to itunes plus

When I try to upgrade previously purchased items to iTunes plus, not all of my purchases are listed. The songs appear on my "purchased" library, but not in the "Upgrade to iTunes Plus" list. Does that mean those songs are not available in an iTunes Plus format? Seems odd, because the information regarding iTunes plus says "All songs in the store are now available as iTunes Plus." And indeed, the song I am trying to upgrade is available for purchase in the store. Why don't all my purchases show up as available for upgrade?