I'm having an issue with my air vents on the passenger side of the dash, it doesn't appear to flow much air unless I close the centre dash vents. I can see the flaps opening in the vents them selves, I can't see under the dash to see if the duct has come off or anything as it runs up over the air bag, I have had the dash facia off to replace globes in the cluster and it all looks ok from there.
Does anyone know if there is a common problem with the duct falling apart behind the dash or internal issues in the heater box with flaps/gaskets that would cause this.

Thanks in advance.