The amount of refrigerant to be refilled into any air con or refrigeration system is measured by weight. Its not by litres or by some pressure measurement. Mechanics use their experience and estimation to what is sufficient. The sight glass at the receiver dryer is only a fair indication of the amount contained. Air con system is also a calculated balanced system of evaporation and condensation. Overfilling or underfilling can result in numerous symtoms of operational defect including compressor malfunction and "not cold". Mechanics can make mistakes to overfill refrigerant especially on complaints of not cold. Refrigerant don't leak out or empty itself unless there's a leak. Its a sealed system as always. In your case it could be a case of fan coil is dirty or need a total washdown, including possible defect of the thermostic control components. Insufficient cooling is also a result of dirty fan coil, where the coil can freeze up, cause the thermostic control to cut off compression on & off (when its melted down) As I said, its a balanced cycle system. Inbalance anywhere cause the thermostatic or protection controls to activate. The same apply to any room airconditioner too...just that its compressor operate on electricity, not car engine.