Problem with heaters not working in part of the house

Hi,
I have an house with a oil heater (kerosen), at the beginning of the winter we noticed some of the radiators were not heating up up stairs and called a plumber ( we blead them first to no avail)
He advised up to reduce the opening of the valve bringing the water on the radiators down stairs to force more water into the rest of the system. This worked.

Today, for non specific reason all radiatirs down stairs this time do not heat up, they are slightly warm at best, all radiators up stairs are fine.

I suspect there is a blockage somewhere, the house is 8 years and this is the first time we have such problem. Could the lime scale in the water cause such problem?

Any idea of what this could be?