Why would the swimming pool heater get hot but not warm the pool water?

Some pool heaters have an internal bypass valve that improves efficiency by limiting the amount of water flowing through the heat exchanger. These can stick so that water flow mostly bypasses the heat exchanger...the heater gets hot but very little heat is transferred to the pool.

Other answerers have suggested the following:

• Your airflow over the pool is cooling the water even as it is being heated
• You may not be running the pool pump long enough. Is the heater not maintaining temp? Is the thermostat turned up? Is the filter clean? When was the last time it was back washed or cleaned?
• You may be using the wrong size heater for your pool and the water to heat ratio that your heater is made to work with may be out.