While working in Quality, I often had to tell people that I respected and admired that their work product was unacceptable. It can be hard to phrase that in a way that does not come across as a judgment.
It was explained to me years ago by a customer that Quality's role is to assist the subcontractor to produce a work product that will continue to earn business. We all have the same goal: a final product which meets the needs of the end user. I worked for that customer for most of ten years, before being hired by the same company in a Quality role.
Those words never left me. As a QA rep I would always try to engage those whose work I was evaluating. "I can tell you worked hard on this product," I'd tell them, "and it's excellent work. I did notice some things that you might have missed though..."
Very seldom did I ever have to "pull rank," which is unpleasant for everyone. I was always gratified to be able to speak to my colleagues and counterparts as equals, and ask, rather than demand, that they make changes I was directing.
Respecting the work other people do is key in that role. You are there to tell someone whose job you cannot do that they need to do it better. It can be awkward. Acknowledge it, and get to the goal that you all share.