My car AC is pretty much useless when idling. It’s cool enough if I am moving down the highway. I have a can of coolant for it, however my gauge says that it is already full. What else might be causing the problem?
The compressor runs off a pulley that turns in time with a belt driven by the motor. It seems to me that the compressor isn’t working efficiently enough at idle to provide a substantial amount of cool air. This isn’t really a problem at speed, for obvious reasons.
Check that the belt is tight and not worn. Now that most cars have spring loaded tensioner arms (and often serpentine belts), loose belts are less of a problem than they once were. Back in the day, when cars had several separate belts, whuch were retensioned by tightening a bolt, a can of belt spray was a routine item in home garages, and you could often test it by simply spraying the belt and compressor pulley. If the A/C went cold, then it was a belt problem. This was good news because it was such a easy fix, even at roadside.
It probably isn’t he compressor. It could be, but there is a much more likely culprit. For the AC to work there has to be a change of state from gas to a liquid in the condenser (in front of the radiator) To get this change of state there has to be air flow across the condenser. No air flow = no change of state = no (or little) cooling.
To make this change of state happen, the car designer puts either electric cooling fans or a fan clutch to make sure the air is pulled across the condenser at idle.
Once you are moving down the road, the ram air effect makes the fan redundant.
So, what I would check is:
Is the electric fan(s) running with the AC on?
Does the fan clutch couple up and move air with the AC on?
The above depend on how the car is equipped
If those appear OK, is the condenser dirty? I have seen dog hair matted into the condenser causing problems.
::: re-reads OP:::
Your gauge says it is full? :dubious:
Despite what you might have read on the gauge package there is no way to accurately service an AC system by just looking at gauge readings. You could very easily have overfilled the system which will give symptoms very similar to an undercharged system.
You might want to consider having the system serviced by a shop with an AC filling station and getting the correct charge into the car. Then if there is still a problem, you can continue down the diagnostic path.
Air conditioning really is not a do-it-yourself thing to work on. About all you can easily do (and in some areas, all you can legally do) is to say “Hmmm… Not working very well” and go to a repair shop that has all of the training, licensing and technology needed to diagnose and service it. If the refrigerant charge is low, laws in many states require diagnosis and repair of the leak before more can be added.
As Rick said, one gauge is not enough - you need to look at both the high and low side pressures simultaneously, as well as the temperature drop across the evaporator and assess if the high is too low and the low is too high, or the high is too low and the low is too low, etc. All of these variations will have different root causes, and most of them are not resolved by blindly adding more refrigerant.
My car is doing EXACTLY what you describe, and when I took it in to be checked, my mechanic told me that the electric cooling fan is only partially working. (It has two speeds and is only working on low.) Since I live in an area where I very rarely sit idling in traffic for more than a minute or so, I declined to have it fixed until the fan is totally dead. In the meantime, I keep an eye on the temperature gauge to make sure the engine isn’t overheating.
Looks like all those years I taught AC didn’t go to waste. 
The solution is obvious, fix your fan.
Also inspect the condenser for dog hair or other materials. A high pressure water hose can dislodge a bunch of crap in the radiator and condenser and might improve things a bit. With the engine off, spray the water from both the back and front side through the condenser and radiator.