I don't know what could be happening.
Try reinstalling it from the console.
Maybe the binary points to dependencies that should be pre-installed.
(There should be no need to move the plugin from .dssi. It works correctly for me as I have indicated.)
sudo apt-get -y install fluidsynth-dssi
I also provide you with this thread where different ways of working with sf2 in different plugins are discussed.
I don't know what could be happening.
Try reinstalling it from the console.
Maybe the binary points to dependencies that should be pre-installed.
(There should be no need to move the plugin from .dssi. It works correctly for me as I have indicated.)
I also provide you with this thread where different ways of working with sf2 in different plugins are discussed.
https://www.rncbc.org/drupal/node/2577