In a newly built home, and I've been noticing that all of the bedrooms seem to be more humid than other rooms in the house, so out of suspicion I put a piece of toilet paper up to the A/C return vents (the ones that suck in air) in those rooms, and the vent won't even hold the paper against the grill. I can't detect even the smallest amount of airflow. What could be causing this? Bad duct design, something else?

Clogged filter is the most obvious cause. Check the filter up in the attic too.

The air filter is clean, recently replaced. So two of the bedrooms are furthest from the air handler, but the closer two rooms I discovered have their air returns jumpered into a hallway return that is doing most of the work. The A/C tech that came to look at it said that that's how they do it based on the homes design now and they used to not put any returns in the bedrooms and I was scratching my head at that statement.

Scratching at which part of the statement? My 90s home does not have returns in the bedrooms. Not ideal, but the way many homes were done.

Door undercuts used to be pretty much standard. See table N3-2 in section 4 of Manual D.

https://hvacrschool.com/what-airflow-goes-around-has-to-come-around/

You should have free air balance's with A/C company for first year. We had to have them come out about 7 times within first year bc they couldn't figure out how to get it balanced properly. Master was always either super hot/cold depending on time of year and front kids bedroom was either freezing in winter or hot during summer due to lack of air. Maybe start with that

Agree with comment on changes to balancing. Typically this is achieved by adjusting air flow at baffles in the attic. A main/trunk line from air handler will connect to a box which then splits air flow to multiple individual registers.

Where the box connects to those lines to individual registers, there is usually a little lever or key on the side of the duct where you can close or open the baffle to restrict or ease air flow to that register as needed. As you restrict output on one line, air flow increases to other lines.

I need to check my attic, but when they came out the first time for a balancing, all the guy did was adjust dampers that are behind my supply vents. It didn't really help much. The dampers are these little metal disks that you can rotate with a stick to restrict or increase airflow directly at the vent exit. I always thought that was not good due to the pressure build-up in the vent run which could cause a burst?

I was always told to never close the supply vents while it is running.

The air will follow the path of least resistance, so my understanding is that as long as it has sufficient alternative paths to flow to, adjusting one damper behind the register shouldn't create too much pressure or be an issue and should work generally the same as I described above.

You definitely would not want to shut all or most dampers because that could increase pressure too much. The air needs to flow somewhere. I would think if you close some some, you'd want to open up others.

I'm not experienced enough to know how to maintain pressure in an optimal range.

I would call your A/C contractor and have them come out. It could be a very simple issue with the dampers. We had a very similar issue and the guy came out and fixed it in just a few minutes. We were getting very little air flow when the A/C was on. They had installed the dampers backwards! i.e. closed when on, open when off.

1. New/different HVAC visit from 5-star rated searches
2. Bill new home builder/HVAC for the fix

Have you been up in the attic to look at the return lines to see if they are "kinked"?

We had a brand new commercial build and they had installed one of the ducts in a way that it was cutting off the flow. They adjusted the ductwork and that took care of it.

The return air will take the path of least resistance, so it sounds like it may be pulling it mostly from the hallway. If you can block up this intake, then it should pull more air from the bedrooms.

But it's just physics that you will rarely be able to feel return air velocity (negative pressure or suction) like you can feel supply air velocity.