The answer to 'why is my water heater won't flush' is that there could be a few common causes, such as a clogged drain valve, sediment buildup in the tank, or a problem with the water pressure. Symptoms may include difficulty draining the tank, slow or no water flow, or discolored water.
To troubleshoot, first try manually flushing the water heater by attaching a garden hose to the drain valve and opening it. This can help clear any sediment or debris. If the water doesn't flow freely, the drain valve may be clogged and need to be replaced. You may also need to flush the entire tank if there is significant sediment buildup. However, if the issue persists or you're uncomfortable working on the water heater, it's best to call a professional plumber to diagnose and repair the problem.