I don't know why this happens, but it often happens to me when I extract parts of my geometry. thankfully it's an easy fix. you can do it one of two ways: one select the object that your having problems with and right click-- go to the bottom of the menu and select matterials, then re-connected the object to your defalt lambert. or , two, open your hypershade select the object with the problem and then right mouse select the default lambert in your hypershade and selcet assign material to selection. and there you go problem all fixed.