As I showed here, grouping seems to be working fine with a default GeneratePress install.
Yes, sometimes plugins and theme conflict (as well as plugins with other plugins). However, GeneratePress is built in a way that should make it compatible with any and all well-coded plugins that are built to work with any theme.
In this case, the theme is working correctly. There is likely something specific to your local install that is conflicting with WP Speed of Light. We’re not aware of the inner workings of that plugin, so I hope their developers/support team are able to figure out what’s going on.