[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.