There is definitely a leak somewhere. You can sometimes see an oily residue left by the gas as it escapes at various joints in the system. Old A/C systems can become porous through the rubber pipes so the system can still lose gas even if everything else is OK.
Mostly when they re gas the system they put in a dye. Then if you have a leak they can use a UV light and you can then see any leak as the the dye shows up when you shine a UV light.
The next time you have it re gassed then ask them to do this
Common issue is a leak on the fill valve. Passes the vacuum test as the valve is open. Fill goes fine , then all the gas leaks away through the valve that doesn't seal. UV dye to confirm.
