Have you tried other wireless tools (e.g. Kismet, airodump-ng, etc) to see how they work? Also, have you tried just plugging your card in & turning on monitor mode from within Wireshark (skipping the airmon-ng step)?
I'm not a Wireshark pro, but I prefer to do my packet capture using something other than Wireshark, then load the pcapdump files into Wireshark for analysis.



