try some debugging:
set security flow traceoptions file SFTPTRACE
set security flow traceoptions file size 2m <====modify file size as fit
set security flow traceoptions file files 10 <===set number of log files as fit
set security flow traceoptions flag basic-datapath
set security flow traceoptions flag packet-drops <===== flag all
set security flow traceoptions packet-filter F1 source-port <> <====could also add filter for source-address
set security flow traceoptions packet-filter F2 destination-port <> <====could also add filter for destination-address
You will learn which policy is causing the packet drops.
take a look at his article, it may also help explain
https://kb.juniper.net/InfoCenter/index?page=content&id=KB19444&actp=search