The answer to 'water heater won't flush' is that a clogged drain valve or sediment buildup in the tank are the most common reasons a water heater won't flush properly. Symptoms include difficulty draining the tank, slow draining, or the tank not draining at all. Solutions may involve cleaning or replacing the drain valve, flushing the tank to remove sediment, or calling a professional plumber if the issue persists.
If the water heater is more than 5-10 years old, it's generally best to have a professional plumber inspect and service it. Attempting to flush an older water heater yourself can sometimes cause more problems, like leaks or damage to the tank. A professional will have the right tools and expertise to safely and effectively flush the water heater.