Hi there,
The CSS for GenerateBlocks is generated by “scanning” the post content of the page.
In this case, the button isn’t in the post content, it’s being inserted by a plugin, so the CSS isn’t being generated.
In GP Premium 1.11.0, you’ll be able to achieve this sort of thing in our Elements module.
For now, you may want to copy the CSS generated by the plugin on the page where you’ve added the button to the post content, then add that CSS to your site manually.