There are a number of ways that you can eliminate geometry from the pipeline before it gets to the pixel shader. The most likely are related to the geometry not being within the clipping volume after exiting the vertex shader. I see some of your vertices after the VS have a w value of > 1.0, which means at least some of the vertices are outside of the viewing area. Double check the output vertices to ensure they are as you expect them to be.
Next could be that the vertex winding is not correct, and the triangles are being eliminated for back face culling. For debugging you can disable culling in the rasterizer state to ensure this isn't the case. This most likely isn't the issue since a cube would have at least partial geometry showing up from either the front or back triangles.
Finally, you have to have the rasterizer generating the pixels to be invoked in the pixel shader. This requires that a viewport be set, and that you aren't using failing the scissor test. Double check that you aren't configuring the scissor test to be active, and that you have set a valid viewport prior to drawing.