Why are some weapon fusions blocked out with a question mark?

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?

Solution 1:

These are weapons that can't be fused at the moment, but will become fusable later. These are tied to "Weapon Unlock" squares in the Treasure Hunt, and maybe also defeating bosses. Zodiac weapons unlock a square that makes them fusable. Beating bosses and other various achievements (e.g. "Acquire 5 Palms") will unlock other weapons.

Whether eventually all combinations become fusable, I don't know - I haven't beaten every boss or found every zodiac weapon yet. But my fusion grid is becoming very dense, so I'm guessing yes.