Do you mean that the air is not getting hot, or just not blowing very hard? If it is not getting hot, that usually points to still having trapped air in your cooling system. when you open your bleed screws (both of them) don't just let it bubble, keep the screws open until you get a steady stream of coolant. Also while the screws are open, squeeze your upper radiator hose to make sure there is no more trapped air.