Every air conditioning system loses a little gas over years and that is normal. A system that went from cold to warm in a few months has a leak, and putting gas into it is the single most common waste of money in this trade.
It is not only the gas. A system that runs low is a compressor running without enough oil reaching it, so the cheap refill in March is part of why the compressor fails in November, and a compressor is not a cheap part.
So the order here is always the same. Recover what is left and weigh it, because how much has gone tells us how big the leak is. Hold the system under nitrogen and watch. If it holds, there is no leak worth chasing and the fault is elsewhere, usually a blocked condenser or a fan. If it does not hold, dye and a lamp and every joint until we have it.
Only then does anything get filled. And we do not sell leak sealants poured into a system, at any price, because what they do to a recovery machine and a compressor is worse than the leak was.