Why does one room feel colder than another?
It's usually not the furnace. It's how air is getting to — and leaving — that room.
When a single room is always colder (or always hotter), the furnace is rarely the culprit. The equipment is producing heat; the problem is getting the right amount of it to the right place.
Air distribution is governed by duct sizing, routing, returns, and something called static pressure — the resistance the blower works against. If a room has a weak return path or an undersized supply, it simply won't receive its share of conditioned air no matter how long the system runs.
Closing registers to 'force' air elsewhere often makes things worse by raising static pressure across the whole system. The fix usually starts with measuring, not guessing.
This article is general education, not a diagnosis of your home. For a proper assessment of your specific situation, talk to us.