Dont mean to sound sarcastic but just because the ductwork is in great shape doesnt mean that it's right. Just to give an example, if you dont put gauges on a unit then you cant properly troubleshoot it because you dont know what is going on inside. Wow, that unit looks like its in great shape!.....lol. I second what Dash said as well concerning static pres.
The whole picture needs to be revealed before any real diagnosis can be made..........l8r