Invisible or tiny
Frame the object and check dimensions. Unit mismatch is common. Determine intended size and use a deliberate scale factor.
Lying on its side
Identify source and destination up-axis conventions rather than repeatedly rotating by eye.
Inside out
Inspect face orientation. Negative scale can reverse winding; correct transforms before recalculating outward normals.
Missing textures
Geometry and images are separate. Locate original images, verify UVs, and reconnect them. Paths from another computer may fail.
Faceted shading
Check smooth shading, split normals, smoothing groups, and sharp edges. Regenerating normals can alter intentional hard edges.