How to capture w60g MAC frame

I used the command:

/interface w60g set wlan60-1 mode=sniff
/tool sniffer set file-name=example
/tool sniffer start
/tool sniffer stop
/tool sniffer save

then get “example” file from web UI, and use Wireshark to open it.
but there are no 802.11 frame package.
How to capture MAC frame from w60g interface?