Please help ASAP. Big system losing static pressure,CFM

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

Are you able to get into the jobsite after hours or on a weekend? You can close the VAV boxes on a floor at a time, starting with the first floor. If you unable to build any pressure on the sixth floor you might look for a blokage or an open.

hi jim , things to look for are fresh air filters? clean? fire dampers shut? fans running the right way<some ex-worker may have swopped 2 ph over < you will have to trace it all out, best of luck .

Hi you should ask to see maintenence reports from last year never belive it when they say all was ok last year if all was ok then it should be backed up with paper work .
Next i would follow the advice as said above messure pressure drop on each floor and try to find where the problem is .
And yes when a small branch taps on to a main branch you should use deflectores .
ALSO YOU SHOULD CHECK MAIN SUPPLY FOR DESIGEN AIR QUANTITY.
Hope this is helpful good luck

Some good points. Thank you for the help. Never even crossed my mind that the motor is running backwards. I get to get in tomorrow morning. I will see if I can close off a couple floors

This is my first time on this support website. You guys are awesome. What an amazing tool to use. What's my best way to measure the air quantity? Anemometer ? I wish you could draw the deflectors to show me how things were supposed to look.

From a contractual point of view, I would organize my own expert "ASAP", do not leave it to the client. other wise you will loose the client.
In the business, nobody knows everythiing, so it is as much about your engineering network. Your airflow expert may need assistance on Chillers.
One presumes you are measuring velocity pressure (air flow), check items as in other threads, you also need to consider how the air is leaving the 6th floor "return air", if these are also blocked or closed, then you can not supply air.
Is it difficult to open/close doors. If so shows a return air problem.
If it has been in for 25years, I can not see your ducting arrangement being wrong (may not be ideal, but thats not the same as not working)
How long have the VSD been installed. If very new, then they could have been set wrong (to fast causing a venturi effect of on first branch)

hi jimmy,
As suggested check dampers and fans. How about vee belts?
Don't assume anything!! always physically check.
I've experience of fans having been running backwards for 15years!
Found dampers seized with actuators slipping on shaft, actuators mis-wired, balancing dampers shut....the list goes on.
We are currently 're-commissioning' all the systems and physically checking everything on a current job.
Could you have dampers stuck open or excessive demand on the lower floors because people have been 'fiddling' with controls?
What is your control system for the VAV boxes and building? Can you set all areas to the same (wide) set points to release any demand and see if the boxes close off all the dampers, then check your duct pressures.