ITunes RIngtone Feature?

I followed the Apple Help instructions on how to create a ringtone for my iPhone by using a song purchased from iTunes. Every song that I bought could not be made into a ringtone. When I chose "create ringtone" a meassage appeared stating that "this song can no longer be made into a ringtone". Then I tried every single song that I have purchased from iTunes in the past month and none of them worked.
Maybe I missed the warning but I didn't see any mention of this in the Apple Help instructions or at the iTunes store before purchasing the songs. So I guess it's just buyer beware? Awesome way to waste money! Please tell me I am totally off base, jumping to the wrong conclusion and that there is a simple solution that I missed. Thanks!