Your hood is very likely recirculating, and that would explain exactly what you are seeing.
There are two kinds and they look identical from the front:
Ducted. Pulls air through and pushes it outside through a pipe. Removes everything - particles, moisture, smells, combustion products.
Recirculating. Pulls air through a charcoal filter and blows it back into the room. Removes some smell. Removes essentially no particulates and no moisture. Very common in flats where there is no external wall route, and it is the default on a lot of cheap installations.
How to tell in thirty seconds: look at the top of the unit. A pipe going into the wall or ceiling means ducted. Vents on the front or top of the hood blowing back at you means recirculating. Also hold a tissue near the outlet while it runs.
If it is recirculating, it is doing what it was designed to do, and it was never going to move your particulate reading.
If it is ducted and still doing nothing, the usual causes are a greased-up filter that has never been cleaned, a flap in the duct that is stuck closed, a duct that is crushed or far too long, or a fan far too weak for the run. All are checkable and the filter is the most likely.