Flat lighting for 3D game with Orthographic camera

Hi,

I'm working on an isometric view 3d game, focussing around a player object jumping up stairs. My camera has been set to Orthographic, and I've selected 'no shadows' under my directional light.

I'm not getting my desired result. The top stairs are rendered differently to the top, middle and bottom stairs. The shading amount varies as the stairs descend. I wondered if there was a setting (for the light and/or materials of the steps) that could ensure they are all rendered equally. The frame is static, no camera movement up or down the stairs.

I've tried rotating the directional light but there is always a spot that seems darker and a spot that seems lighter across the stairs, no matter what angle. Re-positioning the light does nothing. Also, changing the light intensity does not help, as I lose my desired step shade altogether.

The lighting seems to work fine when everything is positioned along the xz axis (flat ground), but because I have steps that ascend it's thrown my lighting right off.

Anything you could share with me to help would be greatly appreciated!

Thanks,

Ben