put your hand inside the vent, about 20 cm there is a metal plate. It could be adjusted. There should be a nob on the side of pipe to control it too. But some time you can't reach the nod because the floor or ceil covers it.
Does every vent in every floor of a house has a metal plate or only some vents have them? I checked the vents in my house, I found some vents have the plates, some do not have. I am considering whether to report this as a problem to the builder.
Thanks.