A hot water heater fails to drain when sediment built up in the heater blocks the drain valve. The best way to get the water flowing is to remove the drain valve so that the water has a larger hole to exit.
Even with the valve out of place, it may be necessary to insert a screwdriver into the hole to break up the sediment. Once the sediment is removed, the water flows out quickly, so it's helpful to have several buckets ready. The water supply is turned off during the draining process, but turning it back on for a few seconds at a time also helps free the sediment. Remember to replace the valve before turning the water back on.