It can be awkward when your client tells you that the delivery team isn’t performing as well as they would like. Instead of being defensive, turn it around.
Two questions to follow up with when a client advises that your delivery team wasn't performing to a level they want from a preferred partner. One's obvious; the other is not.
"What could have been better?"
"What did your current preferred partners do to become great partners?".
These questions show your earnest interest that you want to do good by them and they'll appreciate it. Happy selling...Happy delivery.