Do you have Titan set for either Heel, Steady or Stay? If it's any one of these abilities, it will not attack no matter what until you specifically give it commands to do so. Additionally, if your pet is set for Obey, it will not use AoE attacks until you specifically command it, which means when faced with multiple targets it will usually ignore them to only concentrate on a single target. Try putting Titan into Sic or Guard and see if that helps.