Originally Posted by Pleiades

Sometimes when you pump a tyre up or use a pressure gauge debris or dust can get in and cause the valve not to seat properly. Sometimes the valves just get a bit sticky of their own accord. The next time the tyre is inflated it often dislodges the obstruction and all is good again.

The valve's seating can be unsettled after a long high speed ride especially if the valve spring is a little weak due to centrifugal forces.

Having good quality valve caps with a rubber O-ring seal in them helps prevent dust and dirt causing problems. It's also a good idea whenever you inflate a tyre to quickly release the valve (and some pressure) before you start pumping (or using a gauge) to blow out anything that might be in there.