An AC that runs all day but barely cools the room is frustrating, and it also wastes electricity. The problem is usually one of the following.
Dirty filters and coils
Dust on the filters restricts airflow. Rinse the filters every few weeks during heavy use. Dirty coils need a professional jet clean.
Low refrigerant
Air conditioners do not use up gas. If the level is low, there is a leak that must be found and fixed before refilling.
Wrong settings or room conditions
Check that the mode is Cool, the temperature is set below the room temperature, and doors and windows are closed. Direct sun on the room also increases the load.
Outdoor unit problems
If the outdoor fan or compressor is not running, the AC cannot cool. A weak capacitor is a common reason.
When to call a technician
If cleaning the filters does not help, or you see ice on the unit or hear unusual noises, switch the AC off and book an AC technician.