I just ran across this problem running reaver. I can put the interface into monitor mode, but it does not work correcty. It will only see a few of the AP and it's interrmentent at best.
I put this into my WPS button script and press it before starting reaver.
ifconfig wlan0 down airmon-ng stop wlan0 airmon-ng stop mon0 airmon-ng stop mon1 airmon-ng start wlan0
It seems to fix the problem.