2003 AC issue

My 2003 Venture is only putting out cold a/c on the passenger side and the rear, but the drivers side and the left middle vent blow warm air. The two on the right side middle blow cold. Suggestions?

Yeah-- Classic low charge scenario--- get some gages on it- and add some refrigerant- also look for obvious leaks

thanks brcidd, i added some today. i actually over charged it a little to see if that would help. seems to be blowing a little colder through the driver side. any ideas where the "common" leaks seem to be? also will this get gradually worse as time goes on or do i need to keep adding every year?