Low air flow through the evaporator can cause this problem. Unplug the dehumidifier and remove the water bucket. Remove the three screws on each side of the front cover (see image below). Carefully clean the evaporator coil using a vacuum cleaner with a brush attachment. CAUTION: The evaporator coil fins are sharp.
Replace the front cover and the water bucket. Test the dehumidifier using the high fan speed with the dryness setting maximized.
NOTE: The dehumidifier will not remove moisture from the air if ambient temperature is less than 65 degrees or relative humidity is less than 60%.
Monitor the unit to make sure that the fan is operating and the compressor is starting. If everything seems to be operating normally and the dehumidifier does not remove any water from the air, then you likely have a low refrigerant charge. This type of problem usually needs to be repaired by a service technician.
If you notice a problem such as the fan not running or the compressor not starting, resubmit your question with additional details.