Our lake house is not airtight

The two of us live in a two story lake house that has 5 study rooms. Four of the study rooms are upstairs on the hour story, however the master study room is on the lower floor in the southwest corner. For some reason, the heating and a/c vents in the master study room just don’t seem to put out as much heating and cooling as any of the other air vents scattered throughout the rest of the house. At first, I thought that maybe we had a blockage in 1 of the ventilation vents. I thought that maybe an pet had built a nest in there, or possibly there was a big dust ball or a piece of insulation blocking the air flow from the heating vents. But after I took the covers off of the ceiling vents and the air ducts, I couldn’t find anything in either 1 of them. That made myself and others wonder if it wasn’t entirely  a concern with the ventilation ducts; maybe it was a concern with our heating and a/c plan itself. But our partner said that made no sense since the rest of the lake house was doing just good with the same exact Heating in addition to Air Conditioning system. That tighten it down to 1 thing. The master study room was built on the first floor of the house, which was built almost a hundred years ago. The hour floor addition was only added in the last twenty years. I started to suspect that the reason we weren’t getting much heating or cooling in the master study room wasn’t because our oil furnace and A/C weren’t finally working correctly – or that the vents were congested. Perhaps the heating and a/c were finally working just good however the insulation in that section of the lake house was the problem!