Your car's A/C is a sealed system. Unlike gas or oil, the refrigerant is not used up as you drive, so if the air gets weaker every summer, it is escaping somewhere. The usual suspects are the rubber O-rings at each line connection, the service valves where refrigerant is added, the seal on the compressor shaft, and the condenser up front, which takes hits from rocks and road debris. Heat and age harden those seals, and our long summers keep the system working hard for months at a time.
A can of refrigerant from the parts store may bring the cold back for a few weeks, but it does nothing about the leak, and overfilling can strain the compressor. Some cans also carry sealers that can gum up the parts. The better plan is to bring it to our nearest shop, where we add dye, track the leak with an electronic detector, fix it, then evacuate and recharge to the exact amount your car calls for. Fix it once and the cold air stays.