DIAGNOSIS The most likely culprit is that the exhaust hose is disconnected, kinked, or the window seal is terrible. If the hot air can't get outside efficiently, it gets sucked right back in, and the cooling cycle can't work.
FIX Go through the exhaust path step-by-step. Unplug the hose from both the unit and the window bracket. Inspect it for internal collapses or tears. Reconnect it, making sure both ends click securely into place. At the window, use the included foam seals to block every gap around the slider kit—sunlight around the edges means hot air is leaking back in.
IF THAT DIDN'T WORK The air filter is probably a solid sheet of dust. A clogged filter blocks airflow across the cold evaporator coil. Pull the filter from the front grill, wash it with warm water, let it dry completely, and reinstall it.
HEADS UP These units are single-hose, so they create negative pressure. Some warm air will always infiltrate the room from elsewhere. Don't expect it to cool a large, open space like a central AC would.