If the presta valve with a removable core, remove it, clean it, then re-install it.
If it's a schraeder valve, like a standard car tire, sometimes the valve core can become loose. The remedy is to get a valve core tool and tighten it a bit. I normally remove it completely, inspect the core (sometimes the spring gets hung up or the rubber gasket is split) then reinstall it. Even on new tubes. And dust can work its way in the valve core and wedge it open. I've seen this happen all the time in the Nevada desert. Who knew that a valve cap has some utility?