Things to try when your plugins aren't being called.
Inserting die('loaded plugin')
temporarily in your plugin and die('loaded method')
in the original method (or using debugger breakpoints) is a good idea.
1: Clear the generated files (fast, good for a quick check if your module has already been registered with #2)
rm -rf generated/*
2: Clear all caches and generated dirs, and register new modules. (slow, but basically refreshes everything)
php bin/magento setup:upgrade
3: Regenerate the di code. (slow, but can find issues in your code that the other methods don't)
php bin/magento setup:di:compile
php bin/magento cache:flush
Usually, #1 works, if not, try #2, then #3. If none of those work, check that your module's di.xml
lines up with your modules (in the original post, it seems to).
Double-check that filepaths correspond with class names, and that case matches.
Official documentation for plugins is here: https://devdocs.magento.com/guides/v2.3/extension-dev-guide/plugins.html