The plugin does not trigger a warning or critical status in Nagios

The highest probability cause for this problem is that you have not read the –help output.

Here is the bit you want:
WARNING AND CRITICAL SPECIFICATION
If warning or critical specifications are not provided then no checking is done and the check simply returns the value and any related performance data. If they are specified then they should be formatted as shown below.

In other words, you have to specify some warning or critical criteria on the plugin commandline (-w or -c)!

Here are some examples.