The backlash spring is inside your bobbin case, I would think you'd know if it weren't there but possibly not. I know I dropped my bobbin case and the spring got damaged I wound up buying a new one but managed to fix the first one so now I have a reserve one. I hope you get this sorted it's so annoying when something happens after you've had it in the shop. I would call the manufacturer but I would start with the person who worked on the machine.