Air Return not working

First thing to do is check out your main return. I had a similar problem and it turned out that the main return trunk in the basement had major gaps.

Returns are typically created using the stud cavity on inside walls and they cutout the bottom plate to allow air to flow from the upstairs cavity to the downstairs. Then the bottom plate is cutout of the first floor to get to the basement. Once you have a clear shot to the basement they close off the bottom of the floor joists then mount the main return duct directly to the floor joists making holes in the top where needed. Typically none of these "air ducts" are sealed they just rely on the framing, drywall and flooring to seal them up

My problem was in the basement. The holes cut in the top of the main return duct were too big and some holes went beyond the closed off joists. I fixed it by patching in metal, (one was 4" too big) and filling gaps with caulk or expanding foam. I also caulked around the bottom of the metal plates nailed to the floor joists in the basement. This increased my flow dramatically from the upstairs. I knew I had improved it because when the fan is running I no longer feel a pull on the basement door.

Another thing to look at is where your filter goes. My furnace calls for a 1" filter but the opening is 2" wide, I haven't done anything with this because the improvements made above helped enough. It is probably a good idea to ciculate some basement air anyway.

YMMV. Good luck.

Yeah, you could forget about it!

You didn't say whether lack of air return in these two rooms was a problem, or even noticeable.