You only need about 100 g of CO2 to fully carbonate 10 gal of beer. Also would take several days to full carbonate the beer. Even if you didn't have enough CO2 you would still have pressure in the kegs, just not 12 psi. So you have a leak somewhere - could be the kegs, the hoses and fittings, the regulator or the valve on the tank.
You could tray taping around the top of the kegs so you that you can submerge all the keg fittings with water or starsan to better detect the smallest of leaks from the keg. With regard to the valve on the CO2 tank, generally it will not leak only when it is fully open or fully closed.