Some retailers warn against using a tire pressure guage on a slimed tire, due to slime gumming-up the guage. I suspect that slime has the capacity to similarly gum-up a valve.
You may be able to remove and clean or replace the valve core if the tube has a Schader valve, or one of the removable-core Presta valves (I've never seen one, but I understand that they have wrench flats on the threads that the cap screws onto).