Hi,
I have something funny going on in my model. I have many groups and coloured/materialed, however, when I go to change the colour/material, sometimes the oneclick paint bucket tool works, and other times I have to drill down to the editing group mode to make the change. All the groups are currently in 2D not even 3D yet.
In order to change a material you need to be in the context of the material. Normally all materials should be applied to faces and not the group containers.
Also it’s generally not a good idea to apply materials to your 2D drawing at least if you plan to go 3D. That usually results in more work than if you wait until you have the geometry set up correctly first.
Sharing your SketchUp model file would make it easier to give you guidance.
It is important to understand the hierarchy of material placement.
A material painted on a face will override one painted on a group, and if that group is nested it will override the nesting group.
This gif may be more confusing than helpful, but it shows the hierarchy at work.