Hi JP,
1. For the parameter settings of the Classify Ground Points tool, by default, the "Stop Triangulation When Edge Length<" option is checked. You may uncheck this option and try again. It is expected to get more and denser ground points but the processing time might take longer.
2. There is a tool called Classify Ground by Selected that allows the user to classify selected areas with slopes. You may give this tool a try and see if the ground points on the slope surface can be better classified. If not, please kindly share the data with us and we will give it a try on our end.
Best,
Alvin Qiu