The answer to 'why is my dehumidifier not heating' is that the dehumidifier may not actually be designed to heat the air. Dehumidifiers are primarily used to remove excess moisture from the air, not to heat it. If your dehumidifier is not providing any heat output, this is normal behavior and not a sign of a problem.
Dehumidifiers work by drawing in warm, moist air and passing it over a cold coil. This causes the moisture to condense and be collected in a tank or drained out. The air that is expelled is slightly cooler than the intake air, but it is not actively heated. Some larger, whole-home dehumidifiers may have a heating element to warm the air slightly, but most standard dehumidifiers do not provide significant heating.