Diagnosis
The unit is in 'Continuous' mode, not 'Humidity Control' mode. Or, the room is too large, too open, or has a constant moisture source exceeding the unit's 80-pint capacity.
How to Fix It
Press the 'Mode' button until the 'Humidity Control' icon (a water droplet) is lit on the display. Use the Up/Down arrows to set your desired RH%. The compressor will now cycle on/off to maintain that level.
If That Didn't Work
Calculate the room's square footage. The 80-pint rating is for 2000 sq ft in very damp conditions (90°F/90% RH). In cooler or less humid climates, effective area is smaller. Ensure doors/windows are closed. Check for active water leaks or drying laundry adding constant moisture.
Heads Up
The unit's built-in humidistat has a +/- 5% accuracy tolerance. It may stabilize at 55% when set to 50%. This is within specification.