Return air vents not returning air

Hi everyone! I had a new furnace professionally installed about a year ago. This year I am finishing my basement. There are two return air vents in the ceiling of the room I'm finishing but they don't seem to be doing anything - there's zero air pull coming from them when the fan is running. The upstairs return vents are working (I tested by putting a plastic bag up to them and it pulls it in). So why would the basement ones not be? Are there things I can check without hiring a professional to come look at it? And how critical is it that these work? I'm not sure if they ever have. There are two supply vents in the same room along the floor and those are fine - I can feel warm air coming out of those.

 

Have you confirmed that the ducts are physically connected to the main return line?

The new furnace was installed but did they do anything to the duct work? Maybe it was that way from the beginning?

 

If any combustion vented to a chimney you usually don't want returns in basement...may be disconnected on purpose...?

 

Know that you mention it I dont have any return ducts in the basement!