When did this start? One system for the whole house?

Ducts don't often have all of a sudden problems. They are often inadequate but don't change all that much. Sure they can get a little dusty over time and kinking with flex duct is possible. The other possibility is that a connection develops a big leak but unless this is in an area that you could accidently kick it - it doesn't happen very often.

The biggest question is 1 system or 2. Then - do you have access to the dampers?

I have this problem and it got worse when we replaced the old ball and chain windows with energy efficient.

I think the old windows acted as our return, since we found we don't have one upstairs! Is that crazy or what? Talk about cutting corners! Even stranger it passed home inspection when I bought.

We changed a 10 year old furnace and air conditioner thinking it would help and it didn't..only made it louder.

It helps us to turn up the air and close some downstairs vents. Hope you find the problem.