why wont my truck lift?

ok i have had the rear of my truck bagged for about a year now, and everything is fine back there. But over the winter i bagged the front and i tried to lift the front end for the first time and it wont lift. I used barnyard lower arms and airlift 2500s. I know there is nothing wrong with the valves, becuase i used the same valves to lift the rear and they worked fine. When i try to fill the front bags, i can hear the valve open and let air through, but when go to dump it i only a very little air comes out??thanks in advance