Can you hear the compressor running and not just the fan?
If the compressor IS running, it could be too cold down there which is causing it to frost up rather than sweat like it should or it could be short of refrigerant.
The following link has some details on the latter 2 cases.
My dehumidifier is building up frost on the coils and seems to be running continuously. What might be the problem
The Appliance Information Site
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