The AI analyzes the situation and works in a specific manner. In some turns it may not cast necro call, especially if the target it can call back is not "good enough". For example, the AI prefers to Necro Call the Bone dragons in this fight, then Necromancers, then Vampires. That's the thing one can exploit in this battle. That is also something you can work with - use necro call on the stack that AI values the most and force it to call back weaker units or cast other spells.
EDIT: When the battle is lost and the AI has no units and no mana, it usually stops doing anything as long as there is nothing of relevance to use necro call on. Then it's all about ressing your fallen troops
