Give Permission to Complain
Your customers don't complain enough. (I know, that can feel awfully hard to believe sometimes.) If they did, you'd know all the interactions that frustrate them. You'd know where you've inadvertently forced them through three extra clicks every single time they need to use that feature. You'd understand why they were wary of upgrading to the paid plan and what information would change their minds.
Most people don't complain (to the source, anyways) because they haven't been given permission. They may believe that an awkward workflow is their fault for not better understanding. They may believe that they're making errors because of their own lack of understanding. You need to tell them otherwise. And then you need to put them in a situation where they have no choice, by asking a question with no positive answer options.
- What was the most frustrating part of completing that task? No, please, assuming you HAVE to pick one step, which one did you find most frustrating?
- What do you wish you were able to do at this point, that you're not able to do?
- Other people have said that they were confused by step 2. Did you find step 2 or step 3 more confusing?
- Do you have a friend who has a really hard time learning new technologies? If you were forced to teach him how to use this site, what would you be most worried that he'd stumble on?
People want to make you feel good by giving you the "right" answer. By framing your questions like this, you're making it clear that "no, it's great" is not the right answer. It may be diplomatic, but it's not what you're hoping to hear. And that seems to spur people into shrugging and throwing some honesty your way.