Ok, so that confirms at least that the plugin is picking up the changes in the logfile, so that’s good.
So I’m wondering, from the hostname I’m guessing you access the agent via SSH on this server. What user do you use to connect? If it’s root, no problem, if it’s another user, you don’t have access to the logfile, which would explain this.
Now, on my own server I tried running the agent with the grep command you used:
/usr/bin/check_mk_agent | grep -v grep | grep "<<<logwatch" -A4
Interestingly enough on my server that comes with a complete empty output. However if I run
/usr/bin/check_mk_agent | less
And then search for the logwatch section, I do get it. So, to rule out everything else, add a mathing line to the logfile and then immediately run the agent using the less command. (If you wait too long the host may have been polled by the CheckMK server, and the log entry will not show up anymore).