When do Heroes Cast Spells? Why Aren’t My Heroes Casting Spells?
Heroes cast spells when they have enough mana to do so. Heroes gain mana whenever they deal or take damage. All spells and attacks cause a delay before the hero can take another action. They also have cast times, or the time it takes to completely cast a spell. If a hero is getting all its mana by taking damage, but not giving any damage, it may die before it is able to cast once it reaches full mana.
The best way to make sure your heroes will use their abilities is to be sure all of them are able to deal damage to opponents, and try to split damage around your team by using tanky heroes and clever positioning. Try different field layouts round-to-round and see which formations work with the army you have!
Putting health items on allies, giving them HP regeneration or giving them a void stone will also help them survive long enough to build up their mana reserves and cast spells.