Returned Power Inverter

I returned my Power Inverter as it was faulty to the Zevenwatch Computer Mania store on the 1st of October and they said they would send it to the manufacturer which will take 10 days. It's been more than 3 weeks and I've been to the store 3 times to follow up, and on all occasions I have been told there is no feedback yet. I am quite frustrated as I rely on this device for work purposes, the people at the store don't seem to care at all, they keep saying they'll follow up and update me and I never hear from them! My inverter is still with the manufacture who hasn't fixed after 3 weeks of being returned.