After a long time suffering of sub-optimal fan extraction in my kitchen, I decided to go to the root of whatever was causing it. I have a Zanussi ZHC60136X. I clean the filter grids every now and again (~6 months?) but haven't done any other maintenance on it. I had the impression that the fan was blowing, instead of sucking, and after removing the filters, it's quite obvious that it's venting air out. I did the usual test for this, placing a pot of steaming water on the hob and see if the stream of steam gets sucked when the fan is on: there is not much difference between turning the fan on or off, and some of the steam escapes the hood without being sucked by the fan.
After checking the manual, I saw that there were meant to be some coal filters attached to the air inlet. I thought that the filters must have been saturated and needed replacement, but after disassembling the inlets, I realised that there were no carbon filters at all! There has not been any since I moved into the house (two years and a half ago) and I doubt they were before (the house is around 4-5 years old). The fan seems to work nicely, so the grease has not affected it much even with the absence of filters.
Could it be that the lack of carbon filters is affecting the way the fan works, making it blowing instead of sucking air? Is it too late to fix now, could the vent be blocked somewhere upstream, where I can't clean it? I don't run a chicken shop, the motor assembly was dirty, with some grease as you would expect, but is not a clusterf*** of grease.