Capture problem with iftop or iptraf

Hello,

I want to use a laptop under OpenSuse 12.3 for network monitoring.

I've configured a switch for port monitoring and plug my laptop on the monitor port (I already done this with an ubuntu laptop without any problem).

I'd like to use iftop but I'm encountering a problem:
If I launch iftop alone, it only shows broadcats and local transactions (from and to the laptop).
If I open Wireshark and perform a capture on eth0 at the same time I use iftop, I can see every traffic I want to monitor.

I thought it could be an iftop problem but I tried with iptraf and it's exactly the same.

Well, I think that something is blocked if wireshark is not performing a capture but I don't know where/what to search.

I tried to disable Firewall but it doesn't change anything.

Any idea?

Thank you in advance.