It works for me.
Product Name: NetScreen-NS5GT-ADSL-WLAN
Serial Number: 01234567898765, Control Number: 00000000
Hardware Version: 1010(0)-(00), FPGA checksum: 00000000, VLAN1 IP (0.0.0.0)
Software Version: 5.4.0r10.0, Type: Firewall+VPN
Box in extended mode
www->
www->
www-> snoop detail le
len snoop detail length
www-> snoop detail len
<number> packet length to display (range: 1 - 1514)
www-> snoop det
detail snoop detail configuration
www-> snoop detail
Snoop detail turned ON
My best guess is that you are a read-write admin when you are trying to get detail.
Try it as root admin.
Cheers,