Stopleak basically just swells seals, gaskets and o-rings.
And you can only swell them so many times, so much, before they start to come apart.
Too many folks also "fill" the refrigerant past the proper level, at which point it has no room to expand, and therefore does not cool.
If your compressor takes a dump, you can't just replace it either - you have to do other stuff along the chain - the drier, expansion valves/tubes, and so on. I'd also put in fresh valves, etc. There are also sensors which tell the compressor when to come on. They'll require a vac and fill usually however. If your system gets vac'd, you replace a compressor, etc., make sure the proper amount of pag oil is added - some folks cheap on this, and then wonder why their $300 compressor with a few benjies worth of install just grenaded...
Damn, I almost sound like I know what I'm doing...
For a home AC unit, if you can put the outside part in shade, without tall grass, etc., around it, that will help. Also helps to hose it down a little - maybe at the edge of a lawn sprinkler.