If they can't get it to balance you can have an inline fan installed in the vent that isn't putting out enough air. It mounts in the vent or duct and is electric. It can be connected to run anytime the system does or with a zone thermostat. It is not a cheep solution but a final one. We had to do it in one house we owned. In another we found the duct was blocked and that was causing the problem.
Hope this helps,
Aloha for now,
Trudy
