Diagnosis
The refrigeration cycle isn't kicking in. The compressor might not be starting, or the system has lost its refrigerant charge.
How to Fix It
Check the obvious settings first. Make sure it's set to Cool mode, not just Fan. Set the temperature well below the current room temp. Listen closely when you first power it on in Cool mode. You should hear a distinct click and a deeper hum (the compressor) starting after a minute or so. If you don't hear that deeper hum, the compressor might be locked out.
If That Didn't Work
Unplug it for a full 30 minutes. This lets any internal thermal protection reset. Plug it back in and try Cool mode again. If it still doesn't get cold and you're sure the exhaust is vented properly, it's likely a sealed system issue (refrigerant leak or compressor failure).
Heads Up
Portable ACs are notoriously inefficient if the exhaust isn't sealed perfectly. Even a small gap in the window kit will pull hot air back into the room, making it feel like it's not cooling when it's actually just fighting a losing battle.