I've had the opposite problem with a few presta valves that got so sticky that I couldn't get air into the tire. I'd have to let some air out by hand just to free up the valve. And I agree with Geonz that if often matters where the valve is - the chuck needs to be straight with the valve, so try moving the valve or the pump around until it is. It may also be useful to unscrew the pump head, remove the rubber gasket, and oil it.