1. Here's why
Lumion uses the names of imported materials from Revit, SketchUp, ArchiCAD etc.
If you re-import a model and the imported material names from your 3D modelling application do not match the existing imported material names in your Lumion Scene, the Lumion materials can't be re-applied to the re-imported model.
The result is that the Lumion materials disappear and you only see the original Imported Materials.
1.1: To check if that is the case, please open a version of your .LS Scene file that still has the intact materials.
1.2: Click on the Materials button:
1.3: Move the mouse cursor to one of the surfaces that loses its Lumion material after you re-import the model. Make a note of the name of that material.
1.4: Then re-import the model and make a note of the new material name.
If there is a difference between the before and after names, it means that the names of the re-imported materials have changed.
Below is some advice for Revit, ArchiCAD and SketchUp that will help you troubleshoot this issue and instructions on how to fix missing Lumion materials.
2. Different material names from Revit
Older versions of the Lumion plug-in for Revit used different material names than newer versions of the plug-in.
2.1: For that reason, we recommend that you only import Revit models with the latest plug-in for your version of Lumion and Revit:
Download Link: Download Lumion LiveSync for Revit
2.2: Please also verify that all versions of Revit on all of your computers are using the same version of the plug-in by clicking on the Lumion logo button on the Lumion ribbon in Revit:
2.3: To transfer Lumion materials from a version of the Lumion Scene with intact materials to a version with missing materials, please follow the instructions in section 5 below.
3. Different material names from SketchUp
If you created a new material in SketchUp and did not assign a name to it, a generic name would be assigned to it when you imported it in Lumion.
This generic naming convention was changed in Lumion 8.3.
As a consequence, any Lumion materials that you have assigned to these materials in Lumion 8.0 or older versions will go missing after you re-import the model in Lumion 8.3 and newer versions.
The file formats .SKP and .DAE also use material names that are not compatible with each other.
3.1: For that reason, we recommend that you only import SketchUp models with the .SKP file format. Never switch between .SKP and .DAE format when re-importing models.
3.2: If you are using the Lumion LiveSync for SketchUp plug-in, please make sure that you always use the latest version:
Download Link: Download Lumion LiveSync for SketchUp
3.3: To transfer Lumion materials from a version of the Lumion Scene with intact materials to a version with missing materials, please follow the instructions in section 5 below.
4. Different material names from ArchiCAD
If your Lumion Materials go missing after you re-import a .DAE file from ArchiCAD, you might accidentally have exported your model using the wrong .DAE (Collada) file format, i.e. the Collada file (.DAE) option instead of the Lumion Collada file (.DAE) option:
File -> Save as... -> Lumion Collada file (.DAE)
4.1: Please make sure that you select the correct Lumion Collada file (.DAE) option before re-importing the model.
4.2: If this does not resolve the problem, you can transfer Lumion materials from a version of the Lumion Scene with intact materials to a version with missing materials by following the instructions in section 5 below.
5. How do you fix missing Lumion Materials?
5.1: To get started, please open a version of your .LS Scene file that still has the intact materials.
5.2: Then use the Save and Load Material functions to save individual materials to files:
Knowledge Base: How do you copy, paste, save and load Materials?
5.3: The material files can then be loaded in the version of your Scene with the missing materials.