u/Appropriate-Cover263

My car is a 2016 audi a4 with 115k miles. Check engine light illuminated and I took it to the dealership to have it diagnosed. They found that the turbo was almost entirely non-functional so I had them replace it. The new turbo did function better, I could definitely feel a difference, but the underboost code, and soon after the check engine light as well, returned. They offered to inspect the system to ensure they didn't do anything wrong when replacing it and found no leaks or damage anywhere. The new turbo was producing about 20% less boost than it should have been. They assessed that the new turbo was likely faulty and offered to replace it for free. I took them up on the offer, but the second new turbo performed even worse than before. Has anyone had experience with this kind of problem? Any idea what the cause might be?