Some wireless cards support monitor mode but won't work with
airmon-ng. This is a little script helps you turning the monitor mode on! (+ it also sets the channel and the tx-power)
startWlan.sh [Interface:wlan0] [Channel:1] [Txpower:30] [Bandwidth:HT20|HT40+|HT40-]
./startWlan.sh - enables monitor mode on wlan0, sets channel to 1 and tx-power to 30dBm.
./startWlan.sh wlan1 11 33 - enables monitor mode on wlan1, sets channel to 11 and tx-power to 33dBm.
./startWlan.sh wlan0 6 - enables monitor mode on wlan0, sets channel to 6 and tx-power to 30dBm.