Could it be closed register or damper?

Most common. Walk to the cold room and confirm the register is fully open.

Could it be clogged filter?

Reduces total airflow; the long-run rooms suffer first.

Flex ducts in attics get kinked or crushed by stored boxes. Visual check.

Sometimes ducts pop apart in attics. Heat is dumping into the attic, not your room.

Common in 1990s builds. Static pressure too high, blower can't push enough air.

Troubleshooting

Quick answer

If only some vents are weak, it's a duct or static-pressure issue, not the furnace. Check for closed dampers, kinked flex duct, or a clogged filter.

Closed register or damper
easy

Most common. Walk to the cold room and confirm the register is fully open.
Clogged filter
easy

Reduces total airflow; the long-run rooms suffer first.
Kinked flex duct
moderate

Flex ducts in attics get kinked or crushed by stored boxes. Visual check.
Disconnected duct
moderate

Sometimes ducts pop apart in attics. Heat is dumping into the attic, not your room.
Undersized return ductwork
moderate

Common in 1990s builds. Static pressure too high, blower can't push enough air.

Above didn't help
Static-pressure problem suspected (whistling at the furnace, weak airflow everywhere)

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