Hi @BeakBlack and @noitanigami,
The RootNode material is created when one or more geometries have no material assigned. If you get only the RootNode material it seems that none of the geometries of your model have material (according to the FBX file of course).
What software do you use to export your FBX? If it is 3dsMax for example, you can reimport your FBX file and see how many materials it actually contains.
I suspect that you use materials that are not supported by FBX specification. At the export step, you may have some "material export failed" warnings. Could you check for this ?
You only need to ensure that the option "Show Warning Manager" is checked in the export options (UI dropdown) to get these warnings.