It's a common and frustrating scenario: the AC unit is clearly on, you can hear it running, but the room just isn't getting any cooler. Before assuming the worst, it helps to understand the handful of things that usually cause this.
Restricted airflow is the most frequent cause. A clogged filter, blocked vents, or furniture positioned in front of an indoor unit can all choke off the airflow a system needs to exchange heat effectively. This is also the easiest thing to check yourself.
A dirty condenser coil on the outdoor unit is another common cause. When the coil is coated in dust, pollen, or salt residue, it can't release heat efficiently, which reduces the whole system's cooling capacity even though it's running normally otherwise.
Less commonly, a refrigerant leak or an electrical fault in the outdoor unit can prevent the system from reaching its normal cooling capacity. These generally aren't something to diagnose or fix without a proper inspection, since opening a sealed refrigerant system requires the right tools and handling.