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The most common reason for a L3/L4 capture filter to not work is that the packets are encapsulated at a lower level. Usually vlan tagging. What do you see without a filter when you do a tcpdump -neli <interface>?

If there are vlan tags, try the following filter -f "vlan and host 10.239.81.123".

If that does not work, it would be handy to see a very small unfiltered capture to see what might be in the way.