I bought this 3D model from Sketchfab: https://sketchfab.com/3d-models/robin-bird-0a93168c1a2b4de0a9eebbffc9642d61
This is what the model looks like when I import the GLTF file into Blender

This is what it looks like with the materials

For some reason the eyes seem to be missing. But when I open the .blend file the eyes are there (see image below)

Since I need this 3D model for a project in another software which only supports FBX and GLTF files (GLTF works best) I need to use the GLTF file formar for my project. Why do the eyes disappear? How can I solve this problem?
Thank you very much.