As I recall you get this phenomenon when the Mesh element's 3D geometry is degenerated so ARCHICAD cannot display it in 3D. You still see its nodes when you select it though.
You could either delete it and redo it, or you could find where exactly the Mesh is degenerated. It is usually a single place in the Mesh. I usually do this:
0. Create a duplicate of the Mesh and work with that.
1. Split the Mesh into two Meshes using the Split command. Usually you will see that now one of the two resulting Meshes now displays correctly in 3D. You will then know that the problem is in the other part of the Mesh.
2. Split the problematic resulting Mesh again into two Meshes. Continue this until you can pinpoint exactly where in the Mesh geometry your error is.
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