Your berry filling may be runny if you: - don't use the full amount of cornstarch, - use more berries than the recipe calls for, - use berries that are extra ripe and juicy (add an extra tablespoon of cornstarch), - thaw frozen berries before baking, - use a very narrow deep baking dish, or - under-bake the cobbler. The center of the berry filling must be bubbling (an indication that the temperature has reached boiling to activate the thickening power of the cornstarch.) While the berry base will thicken in the oven as the cornstarch plumps up, it will continue to thicken as it cools.