iPad Apps stuck on "Waiting" while updating

Some of my iPad apps need to be updated through the app store. But now those apps was stuck on "waiting", none of them are downloading though my WiFi network. But my WiFi was working fine. On my Mac, I tried to update the apps through itunes. Nothing happened while I clicked the button to update them.If I turned off WiFi, it started to download no problem. Then I changed the MTU on my router to