Just in case you run into similar cases here. 

A part sits behind or gets enclosed by other components. 

E.g. from the outside, we can't see internal components. 

Such as this retainer highlighted here. 

If we simply drag and drop a material to this area, it won't reach the internal retainer. 

E.g the target was set wrongly to the external Gearbox. 

So one suggestion here is to first select the internal component from the Design Manager Tree, and then drop a material anywhere in the graphics area. Any spot works now that the target has been specified. It won't pick up unintended parts. So your cursor doesn't have to travel a long distance at all. 

Please note, Dropping a material to a tree node doesn't work in my test. Would it be nice if it does?😄

Now, xDesign recognizes that your intent is the internal Retainer and assigns it correctly😉

BTW, it's a good idea to confirm the target part name from the pop-up context bar before dropping a material. 

Of course, you may hide the enclosure to access the internal Retainer directly to assign materials. 

Now these materials will show up nicely in 3DEXCITE Product Communicator