A dehumidifier is nothing more than an air conditioner with the evaporator and condenser coils next to each other and no outside discharge for the heat. In fact the air is pulled in over the evaporator coils then the condenser coils before being discharged back into the room.

Check that the filter on the coils is clean and you get good airflow through the unit. If the filter is clean then the dehumidifier might be low on its refrigerant charge. The units are charged with R22 refrigerant. You might be able to contact an air conditioner service man and get him to check the unit or if still under warranty return it to the place of purchase.

A low charge will cause freeze ups just as in a house hold air conditioner