In the weapon fusion menu, some combinations of items contain an "X" in their location box, and both their name and display image are just quesiton marks (??? and ? respectively). I'm unable to combine these items, but my instincts tell me that there is something more to this.
Is there some restriction that prevents me from combining certain items until later in the campaign (and if so, what is it, and how do I remove it)? Or are some items just not able to be fused?