Need to start with basics like fuel pressure, checking for codes. We can't diagnose it over the internet with little information. If you need a second opinion then use this link to find another shop near you.
https://repairpal.com/repair-shops
Alex
RepairPal Certified shops can diagnose and repair your 2008 Audi A6 Quattro. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.