I would try setting up a tunnel from some other machine so you can tcpdump the remote end. tcpdump should be reliable, so presumably either the packets aren't being sent from the tunnel or something is dropping them along the way. Also, for any network problems I always do netstat -s > BEFORE # ping/etc whatever you are testing netstat -s > AFTER diff -u BEFORE AFTER which often reveals issues I was not looking for.
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