open it up and see if something is jamming the flappers. you can try to cover the output with your hand and pump to see if the out put flapper is jammed, as its normally the pickyest. one small air leak on the output side can shut the whole thing down.
if you try the hand trick and it sucks your hand tight the flapper is stuck/ripped etc.
if you try the hand trick and it sucks your hand tight the flapper is stuck/ripped etc.