Best way to nudge stubborn usability testers?

Here's the situation -

I'm at a new (for me) company and I'm running tests with some users of our product. Due to the nature of our product/users, we can't really recruit so much as we can ask the clients to select users from their small user base for us. From the first session, it was obvious they had come with a list of things they wanted to address, feedback wise. It wasn't ideal, but we did get some good feedback, and I was able to run some task based tests during the session.

The issue arose when administering SUS and NPS questions towards the end. I want to establish a baseline for how UX is performing and (hopefully) improving over time with my involvement, and NPS is important for our CEO, so I need to ask these quick questions at the end of a session.

One of the users was a little hostile to them and pretty reluctant to answer. I explained that we were using a standard scale of questions in order for us to better evaluate the product's performance and give us a baseline to judge against as we continue to help our users(him) through design of new features and improvements. Still, it was like pulling teeth and a bit frustrating, and it took much longer than the few minutes it should have to get these 1-10 answers.

What's the best strategy for dealing with situations with users like this? They may feel expert in their opinion and use, and look down upon this line of questioning. Because of what our product is and the pool of users, I'll likely be dealing with this same participant again for future tests. Anyone had a similar experience?