rear ballast bag not fully draining???

I have a fat seat in the rear of the boat and a 750 bag in the bow; When it comes time to drain the bow does fine cause the water is directed right to the drain fitting but the rear bag does fine until the water is more in the rear of the bag and the pump is just sucking air which doesnt like that. There is no connection in the rear of the bag to ease the problem, I was wondering how do the factory bags drain complete????