There is a little spring pin inside the valve that locks itself in two positions: Deflating Position (air coming out freely) and Inflating Position (100% Airtight). If the air is releasing, that means the valve spring pin may have been left in the Deflating Position. To switch from deflating to inflating, simply push the pin in and turn counter-clockwise 90 degrees so it pops up and the pin will lock itself in a "UP" position and the air will not be able to come out unless you push the pin down again.
(Click to Expand)