Does wireshark show that you have captured the 4 way EAPOL handshake? If not, it will never crack. If you don't have the 4 EAPOL packets from the session, your monitoring wireless interface may be channel hopping and missing the entire conversation. Try to set the channel on your monitoring interface via command line (iwconfig ath0 channel 6 for example) and then force the rekey and see if you have captured the complete session. If so, you should be able to crack the key with cowpatty. Good Luck.