If it changed recently
- Sediment has filled part of the tank, leaving less hot water to draw
- On an electric heater, the upper or lower element has failed
- The dip tube that carries cold water to the bottom has cracked, so cold water mixes into the hot at the top
- The thermostat has been turned down or has drifted
If it has always been this way
A tank sized for two people will not keep up with a family of five, or with a large soaking tub. How fast a tank recovers matters as much as how much it holds.
What you can safely check
Check the thermostat setting. Notice whether the problem came on gradually or suddenly, and whether it started after a change in the household. Flushing the tank helps if sediment is the cause.
When professional help makes sense
Testing elements and thermostats involves live electrical parts, and replacing a dip tube means draining the tank. If the heater is undersized, a plumber can size a replacement or discuss tankless and recirculation options.