monitor traffic interface is showing traffic destined to/originated by routing engine. As you are interested in transit traffic it's not a proper tool.
optput of show security flow session should display session information for both nodes. On secondary node it should have backup state unless you have asymmetric traffiz (Z-mode).
https://kb.juniper.net/InfoCenter/index?page=content&id=KB24407
As milind.mistry@visnet.in suggested you may try to enable trace-options for this flow
set security flow traceoptions file sip_trace set security flow traceoptions flag basic-datapath set security flow traceoptions packet-filter 1 destination-prefix 100.0.55.30/32 set security flow traceoptions packet-filter 1 source-prefix 192.168.1.100/32 set security flow traceoptions packet-filter 2 destination-prefix 192.168.1.100/32 set security flow traceoptions packet-filter 2 source-prefix 100.0.55.30/32
Generate some traffic and then see the trace using
show log sip_trace
Regards, Wojtek