My air supply system is very straightforward, with an extended plenum in the basement ceiling with 13 branch ducts. But, the house has 14 registers. There are 5 registers on the second floor but only 4 branch ducts going up that way. They appear to line up with the registers in the two upstairs bedrooms. In between those bedrooms is the second floor bathroom, which does have airflow from its register but it is by far the weakest in the house. It is also the longest duct line in the house because it goes up to the ceiling.
I see two possibilities: either I've overlooked a very good hiding spot for the branch duct (possibly even disconnected?), or just this one register is split off from an adjacent duct. Which could be most likely?
Other details: -The two adjacent registers, one in each bedroom, have the dampers cut away so they cannot be closed off; I'm not sure why that was done. -All the branch ducts are about the same size, oval 3x7ish. -I recently sealed the air handler and 13 known branch ducts, which did improve air flow throughout the house, but I can't be sure if the same could be said for the second floor bathroom in particular. -House is in the northeast US, built in 1948, second floor was added 1973.