Hard to say without knowing how the system has been installed. I can say though, that I guess it's a "thermo-syphonic" type of installation. (ie, relies on natural rising motion of hot water.) ........ will work a treat for hot water, but for rads, pipework has to be set up very precisely regarding pipe gradients for it to work at all. (In simple terms - the slope on the pipe runs.)
Modern pumped systems have no trouble pumping all around a system.
I assume there is no pump?