​Try this:

ifconfig wlan0 down
airmon-ng check kill
macchanger -r wlan0
ifconfig wlan0 up​

​If that didn’t work then try this:

​ifconfig wlan0 down
​ifconfig wlan0 hw ether [NewMac]
ifconfig wlan0 up

​Then run ​ifconfig ​to check if the mac address actually changed.