2003 Other Holden Models Radiator and Cooling System

My 2003 small car, Holden Cruze, overheated once. The radiator was replaced. Since then, I have a problem of decreasing water in the radiator and increasing coolant in the reservoir tank. 1) the radiator cap was too vacuum, and changed to another one. Still the same problem: every morning I add 80-100ml water to the radiator. 2) I suspected of air-pocket and turned on heater. The result: the radiator cap not as vacuum, but still have a problem of decreasing water in the radiator and increasing green liquid in the reservoir tank. 3) Went back to the mechanic. I was told that there is no faulty in the cooling system and blamed for checking too much (daily) and adding too much water. 4) After 4 days driving without checking it, today I found out that the same problem happens: increase coolant in the reservoir tank. I think there is no losing liquid because there is no obvious leakage. My question is: Why the radiator cannot take back the green liquid from the reservoir tank? What is a possible problem other than thermostat, waterpump, clogged hoses and head-gasket? Why my mechanic cannot diagnose the problem?