Hi,
Thank you for reaching out.
Regarding the “Generate missing Next-Gen version” tool, unfortunately, there is a known glitch where sometimes it’s not related to generating Next-Gen itself, but to that counter, which might be wrong. Our developers are aware and I really hope that it will be improved soon.
However, a way to check that all your images are converted in Next-Gen is to enable the display WebP option(Settings > Imagify), so you can check if there are images not displayed in Next-Gen and if there is a Next-Gen version created for them.
You can check if a Next-Gen version of an image is created if you append .webp or .avif at the end of the image URL.
Also, on the following link you can see how to check if your images are displayed in Next-Gen on your site https://imagify.io/documentation/how-to-check-if-webp-image-is-displayed-on-your-site/.
Let me know if you need further assistance regarding this.
Best Regards,
Saransh.