Using Gems in Combat and Holding Positions

[Fantomen]

1: What spell is he trying to cast? You have to consider the gem cost for casting the spell as well. If the gem cost is 3 then he can´t use any gems to boost his path level and is thus unable to cast the spell. Another explanation might be that you are facing a weak enemy. If the enemy army is judged too weak compared to your own then your mages will not "waste" gems in that battle.

2: I have no idea.

[Illuminated One]

1.) No idea, I think noone really knows that. It could be the AI considered the enemy to weak to waste gems, but sometimes the AI will override spells you have scripted and still waste gems.
Empowerment has nothing to do with it afaik, since I know of cases that worked where spells could only be cast after empowerment.

2.) Try (hold and cast). Or try to let them attack some turns before you cast.