i might be wrong - but i don't think layered shaders are necessarilly viewable in the viewport. it may depend on what version of maya that you're using. i would worry more about how it looks when rendered. it may also be though, that your layered shader is just not set up right or you have a blank layer which is sitting on top of the rest. post up a pic of your viewport and shader network so we can see what's going on.