How do you get feedback from your subscribers?

I launched Outdoor Status a few months ago and it has gained over 350 subscribers. I've promoted the newsletter on Reddit and it has been received well. Engagement is good too, with a ~50% open rate.

Problem: I'm trying to reach out to my subscribers to schedule a 30 min call to better understand their problems and improve the service. Yet, I already sent 40 emails to my most engaged subscribers and I got no responses. I've also proposed to give $20 Amazon gift cards as an incentive.

What am I doing wrong? Is that to be expected? Should I hike up the incentive, to $50 maybe? I feel that I can't make progress without feedback.