Faber Hood Extractor..not extracting?

Faber hood extractor always has looked nice but never has really sucked or extracted as my old one did in another house. Cleaned the aluminium filters, and the ends of the motors but apart from making a lot of noise nothing much gets extracted and even holding a piece of tissue against the filter on full flow will not hold the paper? Hood connected to an outside wall duct and no other motor filters can be seen that need cleaning. OKAY UPDATE! Found the solution, the outside wall ducting flaps were stuck and not operating under the pressure of the fan. Once released and flapping the inside extractor works much better.