VALVE STEMS –

If your tires are flat after sitting for a long time, or even just low on air pressure – be sure the valve stems are straight (pointing directly toward the hub and not leaning in any direction) before you pump them up.

If they are not straight, the metal edge of the wheel rim may cut into the rubber on the valve stem, doing enough damage to cause a leak and make the tube useless! Riding on underinflated tires can cause the valve stem to become misaligned, and also damage the tube.
Air up at the bike project!