You may have a leak due to an improperly seated valve core.
Remedy: Tighten valve core with valve core tool available inexpensively from any car or bike shop. (shown on the image below)
On rare occasion, the valve core sticks in the slightly open position permitting air to escape the tire.
It is very easy to replace valve cores. Simply go to any auto parts store or bicycle shop and pick up a valve core tool and replacement valve core. Using the tool, exchange the new valve for the sticky one and you're done!