You are low on R134, there is a pressure switch that will not allow the AC clutch to kick in unless there is enough pressure. Use, rent, steal, buy or otherwise use a set of gauges and check the pressure. your resting pressure should be around 80psi. If it is lower than I thing 40 it will not work. If it is low add a can, or partial can until you get the proper pressures (40 low side and 240 high side at about 75-80*). Most of the time I find that the schrader valve is what is leaking and it will leak a small amount and over the winter you end up finding out it's below the point where the AC will work.
Jeff
EDIT: If you are using a set of gauges where you actually screw the valve in to open the schrader valve just go to the point where it starts to show up on the gauge, if you screw it in too far it will cause the schrader valve to leak more.
