Ok, lets start with basics...what is the exact issue and when did it happen? Did related traffic work for a long time before this? Lets assume affected IP addresses are 1.1.1.1 and 2.2.2.2...you can try something like below and see what you get. Usually, first packet isnt syn means something related to assymetric routing and connection not completing properly.
fw monitor -e "accept host(1.1.1.1) and host(2.2.2.2);"
fw monitor -e "accept host(1.1.1.1) or host(2.2.2.2);"
fw monitor -F '1.1.1.1,0,2.2.2.2,0,0' -F '2.2.2.2,0,1.1.1.1,0,0' -> this is the idea...source IP, port, dst IP, port, protocol
fw ctl zdebug + drop | grep 1.1.1.1 | grep 2.2.2.2
Let us know if you can do those and send it over or if any info is private, just blur it out.
Cheers,
Andy