Mages casting weak spells

[Yrkoon]

Or maybe the enemy doesn't come within range of your more powerful spells. Happens a lot if the mages are placed at the very back of the battlefield.

Either bring your mages closer to the front line, or get the enemy to close in before starting melee by giving your melee units the "hold and attack" order, or placing them on the very back of the battlefield. That way, the enemy melee troops will get closer to you before the melee starts. Be careful if you use hold and attack though, for your missile units may end up in front of your other troops as they try to get within firing range.