Check the valve and the area where the valve joins the tube for leaks too. Sometimes you have to pump the tube up really big befor an extremely slow leak will make itself known. I had one on an mtb tubethat was so small the air came out one bubble at a time even when the tube was inflated to 3x its normal diameter and held under water. Turned out the cause of the leak was ONE tiny wire in the wire bead of the tire slowly working its way through the tube.

Cheers