I can’t figure out why i can’t capture 802.11 encrypted packets

Hi, i'm on ubuntu 12.04 and i have an issue regarding the capture of 802.11 Wpa encrypted packets. Testing my card i have found that IF the network is unencrypted i can pick up all the packets from all the clients. IF the network is encrypted i can catch EAPOL authentication of other clients and nothing else. My problem is not decrypting the traffic, because i've found that i can decrypt the test file from the Wireshark wiki, my problem is that if the network is encrypted i can't capture packets AT ALL. I see only my local traffic, EAPOL, some ARP packets and broadcasting stuff.

Please someone help.