Yes, what hborre said is one likely cause. Environment props often have the Diffuse_Value and Specular_Value set to zero (or using a black color has the same effect), then the texture is plugged into the either the Ambient_Color or the Alternate_Diffuse. This has the effect of isolating the object and any texture it uses from the scene lighting.

Another cause is, as RobynsVeil pointed out, light pollution from too many lights, or an inappropriate IBL set up can wash out the shadows.