2 downstairs radiators not heating up, though all others are getting very hot.?

we live in a new build (2000April) and has a gas boiler. one of the living room radiators and the radiator in the study room are stone cold despite turning the thermostat to 30C. In the summer, surprisingly all radiators heat up but every winter the problem appears. We called 2 plumbers & one said the 'balancing' is not right. The other said the valves need changed. Can someone help please?