Could be mis-wired and not openng and closig properly ,or at all.Does it maintain the temp.,and respond when there's a call to heat/cool?
Max air flow is worth a try,given what you have stated. You have only two zones,one is kida small,I'd try max.,not default,to allow more air tothe small zone,so the system doesn't have as much excess air to try and deal with.
Oh it responds fine and heats or cools the zone in question. Actually over heats or cools. I think you have nailed it - this zone is too small for the system. There are new errors in the log today citing execess static pressure. So - time for my installer to re-think his duct work.