Update: I've had a problem with my shader experiment too until I checked "Use Tesselation" in my build settings. The problem was that the light was in the center of the room/ floor cube. In the vertex shader of the block none of all 4 relevant vertices was even close to the dynamic light source (in contrast to the floor center), but the pixels in the middle of the cube only got the interpolated small light values from the corner vertices. Thus it helps to activate tesselation resp. decrease the value under "Tesselate shaded". (I was using block geometry, mtl_specBump and 2 dynamic lights, no sunlight (in map properties and sun_light = 0; in script)).