Faber — Duct pipe given of plastic which is coming back in kitchen as we turn it on

I purchased this chimney a week ago and the plastic duct pipe is provided by the company, which is not working as whenever i turn on the chimney the pipe gets air pressure and comes back in kitchen, when i called the technician he told me they have to install aluminium pipe for this, and pay extra, i purchased this chimney a week ago first i paid for plastic chimney as the asked and now they are asking me to pay for aluminium pipe what a fraud is it