If you have cos configured and running 15.1r5, then u seem to be hitting following pr.
https://prsearch.juniper.net/InfoCenter/index?page=prcontent&id=PR571053
To resolve the issue u may refer the following thread..
https://forums.juniper.net/t5/Junos/How-to-avoid-Dropped-packets/td-p/15519
https://prsearch.juniper.net/InfoCenter/index?page=prcontent&id=PR571053
To resolve the issue u may refer the following thread..
https://forums.juniper.net/t5/Junos/How-to-avoid-Dropped-packets/td-p/15519