The temperature your running your dehumidifier in is too cold.
The freon in your dehumidifier changes from a liquid state to a gas state as it leaves the capillary tube and enters the evaporator (the part that gets cold and condensates).
When this change occurs, heat is transferred through convection from the atmosphere (room air) to the freon in the evaporator.
Freon, in a gaseous state, is cold. The transfer of heat causes it to return to a liquid state and the cycle repeats.
The evaporator condensates when it is cold and the atmosphere is warm (doesn't have to be hot). The fan in your dehumidifier draws air across the evaporator, which enables the the unit (dehumidifier) to remove moisture from the air through the condensation/convection (refrigeration) cycle.
If the atmosphere (room temperature) is too cool, the refrigerant (freon) will not heat up (return to a liquid state) and your dehumidifier will not work. It is not uncommon for thick ice to form on the evaporator in this case.
Also, if the closed system (tubes, evaporator, condenser) have a leak, the result is the same and it will not work properly.
A defective compressor may also be the culprit.
Hope this helps.