I am pulling my hair out. I am a service manager of a brand new HVAC company. Most of my experience is with chillers and towers etc.I have a 6 story building. There are (2) 60 ton DX Trane RTU's. The building and equipment are all about 25 yrs old. The 2 units discharge through the roof and then the ducts come together above the ceiling of the 6th floor in the middle of the building. The common supply duct goes down to the first floor. All the floors have branch lines coming off the main trunk. Each floor averages about 15 VAV parallel boxes. I have one floor, the 6th floor, that has no static pressure in its (2) branch lines. My supply trunk is 2" W.C. The VFDs on both units are 100%. When my 2 branches pulls off the side of the main trunk for 6 floor, it immediately drops to 1". After 10 ft it drops to .8 and no box is on it yet. You go 10 more 20 more ft and it drops to .5 then down to nothing. This system supposedly worked last summer. Can one of my fans be overpowering the other? I cut access hole in branch line and didn't see anything blocking ductwork like insulation or something. Do I need a scoop so the air doesn't "rush by" the side branch. Please help, I have 4 employees counting on me and we need this large customer. I am working for a property management company and this facility is a bank headquarters. I've spent a day and a half chasing my tail. I get one more day to figure this out or they call somebody else!! Please help with any suggestions