@flooouu Your screenshot shows that some of the images are displayed in the WebP format, so everything works correctly.
The reports we receive very often do not concern a problem with the plugin operation, but problems with understanding how the plugin works. Below I have attached the tutorial describing how to test the plugin:
https://mattplugins.com/docs/how-to-test-converter-for-media-plugin
Some images are not available in WebP format because when converted to WebP they weighed less than the original ones. This happens with smaller images or previously optimized images.
I also recommend you to try the AVIF format on your website. I did an analysis to learn more about the potential benefits of using the AVIF format for you. Images converted to the AVIF format on your website will weigh 46% less than images converted to WebP only, maintaining even better image quality. This will further improve the performance of your website.