I'd wager the problem here may be trying to use a shared hotbar with one class's Collectable Synthesis on it. Pretty sure each crafting class has its own button you need to use. The skill gets greyed out because you're trying to use the skill that applies to the class it came from, and not the class you're on.

It'd be like trying to share Observe or Basic Touch, etc. Sure, they all get the same skill, does the same thing, but to the game, they're all totally unique skills that only work for the class that learns them.