Modify vertex position using shaders on Windows Phone 8

I am trying to modify the vertex position using a shader. At the vertex offset stage it is used to raise and lower "steps" out of a flat surface. The shader works perfectly in iOS and Android, but completely fails on the WP8 platform. I have taken a look at the platform specific tips on the Unity manual here. However even after that we have the same result. The texture appears fine, but the vertex shader is failing. Any help much appreciated.