dissconnect the air line from the front, take the adapter out and see if you can blow air through it. Then get a small flash light and look down the lower tube to check out the 12g seal that is still supposed to be there. I have seen this seal block the air passage before, but it will usely leak out the bottom when that happens. But do it anyway, as you will need to take this thing apart anyway since it is not working.
Do you have a valve tool?