That would be due to one of three possible issues.
1. Thermostat is stuck open. This should also show on the temp gauge if you have one.
2. Heater core partially clogged. This will cause less hot water to the core, thus less heat from the core.
3. Partially stuck blend door. If the heater blend door in the heater box is sticking, it will not pass all of the air through the heater core, reducing the heating effect.
For #1, replace the thermostat.
For #2, flush or replace the heater core.
For #3, service/repair the blend door in the heater box.
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