We present a comparison of Mo, V and Nb oxides as shell materials atop haematite cores used for selective methanol oxidation. While Mo and V both yield high selectivity to formaldehyde, Nb does not. Very different reactivity patterns are seen for Nb, which mainly shows dehydrogenation (to CO) and dehydration (to DME), indicating the lack of a complete shell, while Raman spectroscopy shows that the Mo and V formation process is not followed by NbO_x. We suggest this is due to the large differences in mobility within the solid materials during formation, NbO_x requiring significantly higher (and deleterious) calcination temperatures to allow sufficient mobility for shell completion.