In the debug log, there should be values given that will be passed to nagiostat. ARe the values valid? If there are no values, then the nagios check is not passing this information, or is not getting a value in the first place. But if in the debug log there is no data whatsoever about this service check, then nagios is not even sending the information to nagiostat. Perhaps a copy/past of the pertinent cfg files, nagiostat.conf will help us work on it.
What I have found out when I have NaN problems, it's found in the debug log. Data will be shown about the host/service/perf data, plugin output, but some times, the plugin data may be blank or invalid data.
One strange thing I've seen with nagiostat is that I have to be carefull how I setup the regex's. Even though I have a unique host/service check combo, it still get's stepped on, by some other unique host/service check.
But yet, one steps on the other, so I have to have
So make sure you aren't getting data from one check, stepping on your data for some completely other host.