I know its hard to believe, but an AC system low on refrigerant will feel coldest just before it starts to fail by freezing up the evaporator in the dash.
Please have the system checked by a professional and don't just use the cans you see at the auto store. Unless you hang a gauge set on the system, you could be missing a key piece of the equation, be it moisture in the system, a partially plugged orifice tube or a failing compressor. Also, many of the over the counter products contain oils, sealants and other crud that can cause more harm than good in a system. If there is a leak, its much better to have a professional recover, fix and then evacuate/recharge the system than just to throw refrigerant at the problem.
-Dan