It is common for a lot of crud to collect on the tank bottom. This crud can clog the drain valve. It is tempting to use air pressure in the tank to blow this crud out. Doing this can lead to all sorts of crud being blown all over the place and under your skin. If the tank has not been drained for a long time then the safest thing to do is vent all the air from the tank, then remove completely the old drain valve. After removal poke a stiff wire into the hole to loosen the crud. And instead of putting back in the cruddy stock drain valve install an elbow, and an extension about a foot long, and a ball valve. After doing this the tank will be much easier to drain. Also, if air pressure is needed to blow crud out, the ball valve is the much safer option. The drain valves that come stock in virtually all consumer grade air compressors rely on a tapered valve seat and valve. This seat is easily prevented from closing completely by the tiniest of particles, particles which are common in consumer grade air compressors, which is what you have.
Eric