The vent is at the bottom of the evaporator inside the freezer at the back.
If you have removed the panels inside the freezer to exposse the evaporator look at the very bottom of this in the middle as this is were the vent is. Ice builds up in there which blocks the air flow so looking at it it all looks clear.
But you need to have a good look under there and feel under there to access the vent as most of it is hidden by the evaporator itself.
Check that and make sure thats all clear then refit the panels and retry it and that should cure the fault.
As on these appliances this is the most common cause for the fault you have.
Let me know what you find.