We have performed numerical simulations to investigate the phenomenon of axial pattern formation exhibited by a non-neutrally buoyant concentrated suspension. Continuum modelling of the concentrated suspension is done using the suspension balance model to identify the underlying mechanism of the phenomenon. We demonstrate that axial concentration variations become amplified to axial bands owing to the influence of the second normal stress difference , and the first normal difference accentuates the effect of. We demonstrate that the end walls of the rotating cylinder are necessary to prevent the smearing out of axial bands but are not a direct cause of the phenomenon.