A water heater may fail to drain due to physical defects, such as a faulty drain valve or incorrectly installed dip tube. Environmental factors, like sediment buildup or hard water, can also contribute to drainage issues. Malfunctioning appliances or frozen pipes may further exacerbate the problem. Regular maintenance, including flushing, anode rod inspection, and drain valve exercises, can help prevent drainage problems. If the issue persists, seeking professional assistance from a plumber or utilizing replacement parts may be necessary.