I am using a plane mesh created with Milkshape and it works fine.
When using a cube may be worth trying to use fitmesh:
"Scales and translates all vertices of a mesh so that the mesh occupies the specified box.
Do not use a width#, height# or depth# value of 0, otherwise all mesh data will be destroyed and your mesh will not be displayed. Use a value of 0.001 instead for a flat mesh along one axis."
When using a cube may be worth trying to use fitmesh:
"Scales and translates all vertices of a mesh so that the mesh occupies the specified box.
Do not use a width#, height# or depth# value of 0, otherwise all mesh data will be destroyed and your mesh will not be displayed. Use a value of 0.001 instead for a flat mesh along one axis."