Cannot assign materials to unwrapped blender meshes

HI guys, I know there are myriad of threads on this subject however I am experiencing an issue here which is driving me nuts and none of the threads seem to explain what is going on here.

I have 7 meshes inside a blender project which have all been unwrapped individually by selecting the object, hitting tab and select all and hitting U. They were then exported as FBX without materials or textures to Unity. Here's the kicker, only one of them can be assigned a material (grass) - the others just turn green. I have been banging my head against a brick wall trying to solve this one, any suggestions?