Diagnosis
Low water collection in a previously working unit is almost always one of three things: a filthy air filter, low ambient temperature, or a refrigerant leak (less common).
How to Fix It
Clean the rear air filter first (see other question). That’s the fastest fix. Second, check the room temperature with a separate thermometer. If it’s below 65°F, the evaporator coils will frost up and the unit will cycle off on defrost, collecting very little.
If That Didn't Work
With the unit running, carefully feel the two metal coils behind the rear grille (don’t touch the fins). One should feel very cold and the other warm. If both feel barely cool or the same temperature, the refrigerant charge may be low, requiring a professional repair.
Heads Up
The 4500 sq ft rating is for ideal conditions (warm, very humid air). In a cool 60°F basement, its efficiency drops dramatically. This is a limitation of all compressor-based dehumidifiers, not a defect.