I have the same problem with my models. I don't know about Revit models, I use Archicad and Blender. I figure that in my model some surface normals are wrong. If material is not two-sided I cannot pick material with the material picker. So if there are surface normal settings in Revit, I recommend you to check your surface normals and fix them in the modeling program, the problem is mostly solved.
Thin objects are the same, even if my normals are correct, I cannot pick materials on them. But I solve that problem as; if I should have a thin object in my model, I create another big object with the same material and export both objects in twinmotion. With replace material, apply any changes in the big object, thin objects material also changes. Then when all my material settings are ok, delete the big object on twinmotion.
Sometimes even if I do all of this, cannot drag and drop other materials on them. I don't know how to solve it properly. Hope you can fix your problem soon. But please if you find any other solution, write it here. I'll be happy to learn it.
Have a nice day
Seyma