How can I fix my Car Heat up problem?

Hi.. I have got Nissan Almera 1.5 CC 2002 model. The car get heat up after few miles run when start from cold and run at 50 mph. Appearantly water from Radiator goes into expansion bottle but it didnot come back into radiator making it heat up. i have to stop on my way to fill the radiator, then it runs smoothly. It also get heat up when drive in slopy area. I have change Radiator cap, thermostat, thermostat gasket, water pump... Can't understand the problem. Please help..