Welcome to the forum.
In CS4, you also must Target your Track. This is new since CS3. That will get things working as you expect.
In all previous versions you also had to target the track you wanted to paste to. That is nothing different. What is different is that with CS4 you can now target multiple tracks at the same time. So be sure to have only the one targeted you want to paste into. You'll also have to update to 4.1, as previous versions had a bug that would only allow a same track paste, even if you did everything right.
Thank you for you'r answer.
Like as you said, i've been targeting the track and only one track at a time but i'll start by downloading the update 4.1 and see what happen. I'll let you guys know it it fix my problem.
I did not recall the pre-CS4.1 Bug, so would never have thought about that.