Be Appropriately Lazy
When I talk to established teams about how to interview existing customers, someone always asks how to follow up on each specific customer's feedback and requests.
My recommendation is: Don't.
Don't ticket individual customers' feedback as you would a bug. Don't prioritize, triage, and close out with detailed resolved or wontfix comments.
Why? Because that's a pain in the ass. And it's sufficiently time-consuming that you'll find yourself -- consciously or unconsciously -- dreading talking to customers. Talking to customers -- the thing that should be a daily or at least weekly habit -- will become a chore that you procrastinate on.
Would customers prefer that you follow up with them personally? Of course. Will some be downright annoyed that you don't. Of course.
But they'll be a lot more peeved if you aren't learning enough from enough customers to better solve their problems.
This doesn't mean you should ask for your customers' time and then leave their comments unacknowledged. It just means you should follow up wisely.
Be appropriately lazy: every month or quarter, write up a summary of what you've learned from your customers and how you're going to take action on what you heard. Bcc it to everyone who took the time to talk with you. Or write a blog post serving the same purpose, and take a few seconds to explicitly point customers to it.
This allows you to close the loop in a scalable way. It keeps the marginal cost of talking to more customers low -- after all, once you write that summary email, it takes essentially the same amount of time to send it out to 20 customers as it would to send to 10.
Sure, the actual interview time to talk to 10 additional customers adds up -- but the conversations are the rewarding part. That's where you uncover insights and identify opportunities. The follow-up part is what most people dread. So stop dreading and start being (just) lazy (enough).