Assuming you mean in hot weather, when you want cooling...

A few things come to mind.

First, does your Auxiliary (electric) fan run? This is easy to check, open hood and look. At idle, it makes up for lack of 'ram' air through the condenser coils in front of the radiator.

Second, is there enough refrigerant?

Third, is it REALLY HOT out? There is a limit to how much cooling to expect at idle.

Check #1 and #2 above.

Others will add to this list.