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Here are some options:
* contact the plugin's author
* check if the plugin has a verbose mode and run it as such
* add debugging to the plugin so you know what is going on
Showing the command and output from the CLI is a good way to communicate the issue:
# su - nagios
$ /usr/local/nagios/libexec/check_tcp -H localhost -p 22
TCP OK - 0.000 second response time on localhost port 22|time=0.000378s;;;0.000000;10.000000