It sounds like you are working with an assembly, and that you have applied a material at the component level. A material assigned to a component will override any materials assigned to entities inside the component. The plugin follows SolidWorks rules for material assignment, which means that:
1. face assignment overrides body or feature assignment
2. feature assignment overrides body assignment
3. component assignment overrides all other assignments
Of course, #3 only applies in assemblies. To manage these assignments, select a face and look in the plugin's Object Properties window:
Near the top, you will see where this face is getting its material from. If there is a material assigned here, then that will be indicated, otherwise, there will be a hyperlink you can use to select which parent entity the face is getting its material from. There is also a green 'up' arrow in the Object Properties toolbar - you can use this to navigate from a face up to its parent feature, and from there to the feature's parent body, and then to the body's parent component, if you are working with an assembly. At each step, you will see what, if any, material is assigned, and you can remove it using the red 'X' button in the Object Properties toolbar.