Shaders are missing from 3D models during texturing usually due to file format incompatibility, incorrect export settings, or broken material links.
Shaders (which define surface light interaction—like shine or matte) need their data stored with the model. Formats like OBJ don’t save shader info—use FBX or glTF instead. If you don’t enable “include materials” in export settings (e.g., in Blender), texturing tools (e.g., Substance Painter) can’t load them. Broken links (e.g., deleting a shader from the tool’s library) also cause loss.
Fix it by re-exporting in FBX/glTF with “include materials” enabled, or re-linking missing shaders via your texturing tool’s asset manager.