flash actionscript: Ok, that?s the case

I have a button that changes the _alpha with the RollOver/RollOut event, it
works fine when I put it alone in the movie

Then I have this "list" wich is a movieclip object with several of that
buttons that change the _alpha.

The problem is that the buttons just don?t work when I give the "list" movie
clip an instance name

In time, I?m usin Flash MX2004

you probably have a path reference problem, but that's about all the help you
can get unless you give some detail like the code attached to your buttons and
path to your object that you want the button to effect