coolant disappearing

for the last few months i have had to keep buying coolant to fill my radiator. i cannot find a leak. i have parked it over cardboard overnight and everything to see where it is leaking but there were no marks. i pressure tested the lines and it has a slow leak because there is a tiny hole in one of the lines but not big enough to let any coolant out. i know for a fact the hole is not the problem. when it runs, the pressure is a little higher than recommended. pressure testing the lines also checks the head gasket i believe and the head gasket is fine. i believe its the thermostat not opening enough but i need some more advice.