So, you have two zone valves and a single circulator? Does the circulator run when only the added on section is calling for heat, or only the zone valve open? Are there drain valves on any/all of the radiators? Have you tried to see if you get any air out of the system there? Are the individual radiators in series or are they on monoflow T's or branches? Depending on the layout of the loops, there may not be enough head to ensure good circulation to all radiators. You might need separate circulators, each triggered by its respective thermostat.
A little more description and maybe a picture or two would help the pros try to help.