You can (from easier to harder):
- Look into the logfile where the reason for the plugin not being loaded is published (and you will find out whether it was found at all and where OpenCPN
looked for it)
- Publish your code so someone can have a look at it
- Step through the code to see where it fails
BTW, the demo_pi code is unmaintained and I'm definitely not sure it is still OK. You should have a look at the code of some real plugins. Nowadays, https://github.com/nohal/launcher_pi
is probably the smallest to start with.