I've noticed that SolidWorks sometimes doesn't want to let me assign a layer to the first item in the feature tree. 

Fixes I have used:

     Re-arrange the order of the items in the assembly to trick SolidWorks into accepting the layer assignment

     Expand the item and assign the layer to the items inside it 

     Put something small at the top and ignore the issue

Any ideas why this is? Are there other work-arounds or fixes for this issue?