Diagnosis
The most common culprit is an incomplete seal on the window vent kit. If hot air from outside is getting sucked back into the room, you're just moving air, not cooling it. The second likely issue is a dirty condenser.
How to Fix It
First, check your window setup. Feel around the adjustable window panel and the hose connections for drafts. Use the foam seals that came with the kit and maybe add some painter's tape temporarily to seal any big gaps. Second, make sure the exhaust hose is as straight and short as possible, no kinks.
If That Didn't Work
The unit might be low on refrigerant, which is a sealed system. You can't recharge it yourself. If sealing everything perfectly doesn't help, there may be a leak, and you'll need to contact support.
Heads Up
These portable units have to work extra hard. Even a small gap in the window seal can cut cooling efficiency in half. It's all about keeping the hot air it generates out of your room.