Root Causing Tri-level Home Heat Issue

[k5775k]

I have a tri-level home with a sub basement.
-Floor 3 (top floor with bedrooms) sits over Floor 1 (garage, family room, laundry & HVAC).
-Floor 2 (living room, kitchen and dining room) sits over the sub basement.

HVAC has 2 distribution trunks (A, B), with no trunk dampening but has necessary HVAC returns on Floor 1, 2 & 3.
Trunk A feeds floor 3 (4 bedrooms) and some floor 1 space (laundry room, bathroom) next to the HVAC unit via 6” takeoffs.
Trunk B drops down into the basement and feeds Floor 2 with several 6” takeoffs and one 8” takeoff that circles back into the slab for Floor 1 family room which has 3 registers in series.

All registers work well except the 8” takeoff that feeds 3 registers in the Floor 1 family room slab. The registers in the slab are useless. I have looked inside and found no blockage. Not sure what the problem is? All other 6” takeoffs from that same trunk have good airflow.

I understand the chimney effect in a tri-level, being on a slab and two exterior wall makes the family room cold.... but I hardly get any heat from the 3 registers in series coming from the only 8" takeoff in the entire system

[pecmsg]

Tri levels are the hardest.
Static pressure tests to see what the ducts can handle.
Maybe a small mini for the basement.